That claim appears to be apocryphal. It can be surmised that the average ages of men serving on the front lines in both the Vietnam War and World War II were older than 18, but younger than 26. An average of around 20-25, leaning to the lower end, is a reasonable estimate assuming that casualty distributions roughly mirrored the number of men of each age sent to war.

Of the 58,256 Americans listed on the Vietnam Veterans Memorial, the average age is 22.8 years old. The youngest, Marine Private First Class Dan Bullock, was only 15 years old, lying about his age to enlist. The oldest man listed on the wall is 63 year old Army Sergeant First Class Dwaine U. McGriff. McGriff survived serious injuries in 1970 at the age of 35, but died in 1999, never fully recovering; men whose deaths are deemed to have been caused by war-related injuries are entitled to have their names put on the wall as well. As many as 38 men are believed to be mistakenly listed on the wall because of clerical errors, although the exact number is not known.

In another estimate, the Department of Defense's Combat Area Casualties Current File, with 58,169 names (58,148 with both birth date and date of event), says that the average age of those killed in Vietnam was 23.11 years old. About 24% of those killed were 20, with the next most common age (17%) being 21. It can also be noted that well over 1,000 men over the age of 40 died.

Selected CACF Figures:

Type	Number of Casualties	Average Age
Total	58,148	23.11
Enlisted	50,274	22.37
Officers	6,598	28.43
Warrant Officers	1,276	24.73
E-1 (Private)	525	20.34
11B MOS (Infantryman)	18,465	22.55

The figures for WWII on the ages of fighting men mirror those from Vietnam. American soldiers again varied widely in age. Some were 17 year old volunteers, others were 35 year old draftees, but most were in their early to mid-20s. The 1994 journal article Why Do World War II Veterans Earn More than Nonveterans? by Joshua Angrist and Alan Krueger sampled 306,979 men for the table on page 80. Men born in 1922 (19-23 years old from 1941 to 1945) would be the most likely to have served in the military in WWII, with men born in 1921 (20-24 years old) being the second most likely.

Birth Year	% of Veterans in Each Sample
1919	73.2
1920	75.5
1921	77.9
1922	78.0
1923	76.6
1924	75.9
1925	75.0
1926	73.9
1927	65.6
1928	30.8
1929	12.2

In October 1943, the age distribution of the 6,862 men in the 10th Light Division (Alpine) while training at Camp Hale, Colorado was as follows. The most common age group can be seen to be 20-22, with those 18-19 second, and those 23-25 third.

Ages of Men in the 10th Light Division (Alpine), October 1943:

Group	Number	% of Men in Each Age Group
18-19	1,493	21.8
20-22	2,141	31.2
23-25	1,167	17.0
26-28	839	12.2
29-31	580	8.4
32-34	313	4.6
35-37	262	3.8
38+	67	1.0

Most of the original soldiers in the experimental battalion that would become the 10th Mountain Division had been selected or volunteered because they showed proficiency in skiing or mountaineering. Other soldiers came from the 3rd, 41st, and 44th Infantry Divisions. As not enough experienced men could be found to make a whole mountain division, soldiers were transferred from the 30th, 31st, 33rd, and 89th Infantry Divisions. Other fillers came from inactivated armored infantry or tank destroyer units.

A snapshot of the 192 members of Company L of the 134th Infantry Regiment, 35th Infantry Division in Camp Kilmer, New Jersey, in May 1944 in combination with enlistment records from 1938 to 1946 can be used to give a good sample. The 35th Infantry Division was a National Guard division made up of troops from Nebraska, Kansas, and Missouri; the 134th Infantry Regiment was raised mainly from eastern Nebraska, and Company L came from Omaha.

Soldiers who had served in the Regular Army or National Guard before WWII tended to be slightly older than what could be considered average. Promotions in the small pre-war U.S. military were often few and far between, and it was not unusual to find a private or private first class born in 1908 or 1909. As National Guard units could rarely be considered complete as per the TO&E and combat ready with just the men who assembled at the town armory, they needed to be filled up with draftees and volunteers. The addition of large numbers of non-local men as well as turnover meant that many units only ended up being National Guard in name and tradition only; the 30th Infantry Division had 100% turnover in 1942 alone.

A common Army practice when filling units with men in an efficient manner was taking draftees or volunteers from reception centers near where the unit was activated or where they were going to start their training. The 4th Armored Division, while stationed at Pine Camp, New York, received large numbers of draftees from the mid-Atlantic states. The 35th Infantry Division, after its federalization on December 23, 1940, moved to Camp Joseph T. Robinson, Arkansas, to begin training. 62 of the 149 draftees from the Company L roster can be seen to have Army Serial Numbers that begin with "17" or "37," indicating that they volunteered or were drafted from the Seventh Service Command, comprising the states of Colorado, Iowa, Kansas, Minnesota, Wyoming, Missouri, Nebraska, North Dakota, and South Dakota. This resulted in an interesting situation where a man from Nebraska, Kansas, or Missouri, if he was drafted or volunteered at the right time, could find himself serving in his "local" unit even though he had never been a member of the National Guard to begin with.

Men of Company L by Origin, May 1944:

Number	Description
87	Draftees not from the Seventh Service Command
62	Draftees from the Seventh Service Command
29	Federalized National Guard soldiers from Nebraska
3	Volunteers from the Seventh Service Command
3	Volunteers not from the Seventh Service Command
3	Federalized National Guard officers from Nebraska
3	Federalized National Guard officers not from Nebraska
1	Federalized National Guard soldier not from Nebraska
1	Pre-war Regular Army volunteer

The draft age was lowered to 18 in November 1942, and men of 18-20 soon became the predominant group called to serve, often being drafted out of high school. By mid-1944, 50% of all men received by the Army were 18 years old. 50% of Army men went to the Army Ground Forces, and 90% of those were assigned to replacement training centers, with 80% of them going to Infantry or Armored Force RTCs. Due to a manpower crunch which began in the Army in late 1943, fathers began to be drafted (many of whom had children born before Pearl Harbor and were over 30) and men with manageable sexually transmitted diseases, as well as mild physical and mental disabilities, were accepted beginning in early 1944. As a result of the receipt of many older or less physically qualified men, rejection rates at replacement training centers for unsuitability reached an all-time high in fall 1944. It was not uncommon to find 30 year old replacement privates with two or three children back home by winter 1944 as the manpower crisis began to bite.

Most of the men serving with Company L in May 1944 were born in the late 1910s or early 1920s, with a few exceptions, As can be seen in the charts below, the average age of the draftee was lower than that of the federalized National Guard soldier, ~20-24 versus ~25-26.

Ages of 6 Federalized National Guard Officers of Company L, May 1944:

Birth Year (Age)	Men
1913 (31)	1 (2nd Lt. John Campbell, O-1325554)
1921 (23)	1 (1st Lt. Francis S. Greenlief, O-1291268)
Unknown	4 (1st Lt. William D. Brodbeck, O-416863; 1st Lt. Dwight F. McCollister, O-1286325; Capt. James Lassiter, O-1290769; 2nd Lt. Lewis E. Dailey, O-1325357)

Ages of 29 Federalized National Guard Enlisted Men of Company L, May 1944:

Birth Year (Age)	Men
1906 (38)	1
1914 (30)	1
1915 (29)	1
1916 (28)	2
1917 (27)	2
1918 (26)	4
1919 (25)	9
1920 (24)	3
1921 (23)	5
1922 (22)	1

Ages of 69 Drafted Enlisted Men of Company L, May 1944:

Birth Year (Age)	Men
1906 (38)	2
1907 (37)	3
1908 (36)	2
1909 (35)	3
1910 (34)	3
1911 (33)	4
1912 (32)	1
1913 (31)	1
1914 (30)	2
1915 (29)	3
1916 (28)	1
1917 (27)	3
1918 (26)	1
1919 (25)	1
1920 (24)	5
1921 (23)	8
1922 (22)	7
1923 (21)	6
1924 (20)	9
1925 (19)	3

The ranks of Company L would be torn asunder within days after it entered combat, being subject to the horrific attrition that characterized infantry fighting. By the end of the war, 44 of the 192 original men that crossed the Channel with it had been killed or died of wounds, and just 11 didn't become casualties.

Month	KIA	DOW	SWA	SIA	LWA	LIA	MIA	Sick	NBC	Total
7/44	37	3	16		112	5	8	12	3	196
8/44	9	1	6		36	7	9	19	1	88
9/44	15		2	1	30		19	2	1	70
10/44	3		1		3		18	3	2	30
11/44	24		12		124	8	2	20	43	233
12/44	8		13		46	7	111	14	3	202
1/45	6		3	2	16	20	1	13		64
2/45	2		2		5	4		7	1	21
3/45	1				28	1		9		39
4/45	1				4	1		10		16
Total	106	4	55	3	404	53	150	109	54	959

A mildly paranthetical, but still related point: I believe the voice reciting the bit on relative ages in conflicts on the song 19 was a sample of Will Lyman, the narrator for Stanley Karnow's VIETNAM series, which aired in the early 80s.