I second u/jewishobo's recommendation of my previous answer.

Was it common for men in their early 40s to early 50s to be in such active combat roles in the US military during WWII?

They were definitely a significant minority, owing partially to Army personnel policies regarding older enlisted men, although it did happen; men in their 40s and 50s near the front were more likely to be officers such as majors, lieutenant colonels, and colonels.

The greatest number of inductions and voluntary enlistments into the Army from November 1940-June 1945 (inductions November 1940-June 1945, and voluntary enlistments July 1941-June 1945) by birth year, 714,420 or 7.5% of the total, came from men born in 1921, i.e., aged 20 years and 6 months to 24 years and 6 months old.

The greatest number of inductions by birth year, 553,107 or 7.0% of the total, was among men born in 1919 (21.5-25.5), while the greatest number of voluntary enlistments by birth year, 203,995 or 12.6% of the total, was among men born in 1922 (19.5-23.5).

149. Army Inductions and Enlistments by Year of Birth Through June 1945

Year of birth	Total	Percent	Inductions^1	Percent	Enlistments^2	Percent
Total	9,567,405	100.0	7,952,606	100.0	1,614,766	100.0
Before 1905	372,058	3.9	318,003	4.0	54,055	3.4
1905	100,608	1.0	92,418	1.2	8,190	0.5
1906	139,472	1.5	129,536	1.6	9,936	0.6
1907	169,182	1.8	155,964	2.0	13,218	0.8
1908	182,049	1.9	167,000	2.1	15,049	0.9
1909	197,306	2.1	180,559	2.3	16,747	1.0
1910	223,766	2.3	204,506	2.6	19,260	1.2
1911	236,756	2.5	215,618	2.7	21,138	1.3
1912	274,468	2.9	248,661	3.1	25,807	1.6
1913	301,817	3.2	270,982	3.4	30,835	1.9
1914	351,170	3.7	311,107	3.9	40,063	2.5
1915	411,104	4.3	357,867	4.5	53,237	3.3
1916	472,382	4.9	403,763	5.1	68,619	4.2
1917	532,715	5.6	448,476	5.6	84,239	5.2
1918	635,805	6.6	530,023	6.7	105,782	6.6
1919	678,664	7.1	553,107	7.0	125,557	7.8
1920	681,298	7.1	518,197	6.5	163,101	10.1
1921	714,420	7.5	531,347	6.7	183,373	11.4
1922	701,821	7.3	497,826	6.3	203,995	12.6
1923	674,229	7.0	521,023	6.5	153,206	9.5
1924	616,766	6.4	523,576	6.6	93,190	5.8
1925	414,719	4.3	366,028	4.6	48,691	3.0
1926	390,186	4.1	322,798	4.0	67,388	4.2
1927	94,344	1.0	84,221	1.0	10,123	0.6

NOTE.--Data are for United States and territories.

1 Inductions November 1940-June 1945

2 Includes (a) Enlistments July 1941-June 1945 and

(b) Enlisted Reserve Corps called to active duty July 1941-June 1945

Excludes (a) Commissionings direct from civil life and (b) WAC’s.

Large numbers of men being inducted by Selective Service, particularly in the last half of 1942 when induction calls were the highest of the war and prior to the military lowering the maximum age of acceptability by decree from 44 to 37, created some interesting situations:

Along the dusty road to Camp Gruber, forty-four year-old Pvt. William S. Frederick, Sr., encountered his own son, twenty-year-old Pvt. William S. Frederick, Jr. Both were taxicab drivers from Olean, New York; neither had dependents; and both had been snapped up by their local draft board.

Rejection rates for military service were directly correlated with age. For example, between September 1942 and June 1943, 36.4% of all Selective Service registrants were rejected, either at their draft board during the classification process when a cursory examination was performed that checked for one or more "manifestly disqualifying" defects, or at a military induction station, where a full, comprehensive examination was given. Rejection rates varied between 25.2% among 19 year olds, to 67.5% among 45 year olds. Interestingly, the rejection rate among 18 year olds was slightly higher than 19 year olds. This was accounted for by the voluntary enlistment of many physically able 17 year olds in the Army Air Forces or the Navy before they were required to register for the draft which lowered the number of fit 18 year olds, and the fact that of those 18 year olds who did register, some were subsequently rejected for "physical and mental immaturity."

There was a concerted effort by the Army beginning in December 1942 to obtain discharges for enlisted men aged 38 and over if it was found that, "by reason of advanced age" they were unable to satisfactorily perform military service but were able to take jobs as civilians that could assist the war effort. The reasoning of being handicapped by advanced age was later deleted, and all enlisted men 38 and over could request discharge by their own application. Men stationed overseas could also request discharge, but only if trained replacements were available for their positions.

War Department Circular No. 397, 7 December 1942:

II. 1.) Certain enlisted men who by reason of advanced age, 38 years of age and over, are unable satisfactorily to perform military service, but who are qualified to assist the national war effort if discharged from the Army.

2.) Authority is granted by this circular to discharge such men for the convenience of the Government, subject to certain limitations.

3.) The basis for decision by the officer having discharge authority to discharge such an enlisted man will be that the enlisted man-

a. Has voluntarily requested discharge in writing to his immediate commanding officer.

b. Is handicapped by advanced age, 38 years and over, to such an extent that his usefulness to the Army is secondary to that of industry.

c. Has presented satisfactory evidence that he will be employed in an essential war industry, including agriculture, if he is discharged from the Army.

War Department Circular No. 39, 4 February 1943:

II. Discharge of enlisted men 38 years of age and over.-Section II, Circular No. 397, War Department, 1942, is rescinded and the following substituted therefor:

1.) Enlisted men stationed in the continental United States.-a. Within the continental limits of the United States, enlisted men 38 years of age and over on active duty may be discharged for the convenience of the Government, subject to the requirement that the individual enlisted man has-

(1) Attained his 38th birthday on or before February 28, 1943.

(2) Enlisted or was inducted into the Army on or before February 28, 1943.

(3) (a) Prior to May 1, 1943, voluntarily submitted to his immediate commanding officer written request for discharge. Failure...to submit written request prior to May 1, 1943, will forfeit any right to discharge....Note that the limiting date pertains to the submission of the application. Actual discharge may be effected at any convenient time subsequent to submission of application, or may be indefinitely postponed when the interest of the service so dictate.

(b) Commanders having discharge authority are authorized to extend the time for submission of application in individual cases when the enlisted man did not have reasonable opportunity to file his application (for example, due to absence on furlough).

(4) Presented evidence...from a prospective employer, the United States Employment Service, farm agent, or other responsible person to the effect that the individual concerned will be employed in essential industry, including agriculture, if he is discharged from the Army.

/u/the_howling_cow has given you a couple terrific answers about overall age distribution, but I want to chip in here briefly - especially since there's something I've wanted to cheekily use as a source for a while and your second question provides an opportunity.

There's a particularly telling answer when it specifically comes to D-Day and age: Ted Danson at 55 portrays a captain. This was theoretically possible given some of the activation of the reserve and National Guard officers that the Army had insisted for years prior to the war would serve as the nucleus of the officer corps during wartime expansion - a tiny handful even dated back to the World War I Plattsburgh camps, although an attempt to revisit Plattsburgh training for both them and a new generation in the late 1930s fell flat on its face - but was extraordinarily unlikely for a frontline combat role. (If you're really interested, Spencer and Clifford's The First Peacetime Draft talks about how the army got into this position with their prewar stance on the officer corps.)

We do know the age and identity of the oldest participant in the landings, who was a year older at 56. That was Ted Roosevelt, Jr. - the son of Teddy - a brigadier general who really shouldn't have been participating as his health was already suspect (he was walking with a cane, had concealed heart problems from superiors, and was dead of a heart attack a month later) but whose leadership at Omaha Utah itself was nothing short of remarkable. But that should give you an idea of officer age distribution versus rank; there were indeed 50 somethings and even older fighting, but in 1944 they were generally senior officers like Eisenhower (54), Patton (59), and Bradley (61).

You were far more likely to see 50-something midlevel officers in the Navy. That service had two requirements if you wanted to stay in at the O-3 through O-5 ranks during the interwar period. First, you needed an officer evaluation of at least 'fitted and retained,' a death sentence for any chance at flag rank (and at the time even command) but a judgment that the Navy still had some use for you if you accepted that your career wasn't going anywhere. Second, you needed to be under 64. This latter age limit was servicewide and almost never waived, and one of the more amusing applications of it was when Ernest King nervously wrote FDR in 1942 as he approached mandatory retirement, "It appears proper that I should bring to your notice the fact that the record shows that I shall attain the age of 64 on November 23rd next - one month from today." In a classic FDR move, he cheekily sent the letter back with a brief note on the bottom: "So what, old top? I may send you a Birthday present! - FDR," which was how King learned he'd continue to command the Navy for the duration of the war.

That cadre of 'fitted and retained' officers that had stuck through limited promotion opportunities, though, proved quite valuable to the Navy at the outbreak of the war. The vast expansion of the Navy meant there were plentiful commands now available that weren't particularly glamorous - destroyers, minesweepers, supply ships, and later in the war destroyer escorts and LSTs - and didn't offer fast track promotion opportunities, but did need experienced skippers and XOs who'd stuck around through the 1920s and 1930s to lead the activated Reserves, 60 and 90 day wonder officer candidates, and enlistees crewing them. C. S. Forester deliberately wrote the Captain Krause character Tom Hanks adapted for his role in Greyhound as part of that cohort, and oddly, it was probably more age appropriate for him to play the 1942 CO of a Navy destroyer at 64 than Captain Miller in 1944 at 42!

Last, to your question about becoming an officer, since it's an aside I won't get into all details like the -wonders, Air Corps blue bars, the V-12 program, and a few others. Instead, I present to you the most famous mid grade officer of them all nowadays, one Richard Winters (who found himself promoted to captain at 26 and major at 27) and part of his experience that gets discussed far less than Normandy.

Winters had graduated from Franklin and Marshall College and enlisted as a private for $21 a month to get ahead of getting called up in the draft, thinking there wouldn't be a war and he'd get his required year of service out of the way earlier. Winters had been retained as a corporal at the end of basic training to train the next group - his class had been sent to Panama - and with his degree got to volunteer for officer training school at Fort Benning. 90 days later, he's a second lieutenant, and that is where his story generally begins when he volunteers for the paratroopers.

But why did Winters volunteer for OTS? I'll let you listen to him directly.

Yep, "As I looked at (the officers assigned to basic training) I thought, I'm a better man than these fellas. I know I could do better." Never let it be said he lacked confidence!

Here is a related question asked here previously https://www.reddit.com/r/AskHistorians/comments/7825zd/in_world_war_ii_the_average_age_of_the_combat/ including a great response from /u/the_howling_cow