My question is the same as in the title. I know that the average age was 26 over the entirety of the war but specifically I would like to know if and how this changed over the course of the war.
This is combined and expanded from a few answers here and here. The U.S. Army suffered from allocative inefficiencies beginning in 1943 that were cured partially by inducting men in their late 20s who had children; the proportion of these men as a part of the total inducted was thus higher than it would have been had the practice been started in say, 1941 or 1942.
As I talk about in this post, the average age of U.S. combat soldiers in World War II being 26 versus those in the Vietnam War being 19 is probably apocryphal; the average age of those men killed (assuming the distribution of men killed matches the proportion of each age group sent to war) in Vietnam was closer to 23. I would not expect that big of a difference over the entirety of WWII, considering the total mobilization of America's manpower for the war.
The enlistment records of 147 of the 193 enlisted men (76 percent) that made the ultimate sacrifice on June 6, 1944 are available through the National Archives. 1 man was a federalized National Guardsman, 40 voluntarily enlisted on or after July 1, 1940, while 106 were drafted.
Age of 1 Federalized National Guardsman of the 16th Infantry Regiment, 1st Infantry Division, June 6, 1944:
| Birth Year (Age) | Men | Ranks |
|---|---|---|
| 1917 (27) | 1 | Pvt. |
Ages of 40 Voluntary Enlistees of the 16th Infantry Regiment, 1st Infantry Division, June 6, 1944:
| Birth Year (Age) | Men | Ranks |
|---|---|---|
| 1910 (33-34) | 1 | S/Sgt. |
| 1915 (28-29) | 1 | Pfc. |
| 1917 (26-27) | 1 | Sgt. |
| 1918 (25-26) | 3 | 1 T/5, 1 Sgt., 1 S/Sgt. |
| 1919 (24-25) | 4 | 1 Pvt., 1 Pfc., 1 T/5, 1 Sgt. |
| 1920 (23-24) | 5 | 2 Pvt., 1 Sgt., 1 S/Sgt. |
| 1921 (22-23) | 8 | 1 Pvt., 4 Pfc., 1 Sgt., 1 S/Sgt., 1 T/Sgt. |
| 1922 (21-22) | 11 | 2 Pvt., 5 Pfc., 1 Cpl., 1 Sgt., 1 S/Sgt., 1 T/Sgt. |
| 1923 (20-21) | 4 | 3 Pvt., 1 Pfc. |
| 1924 (19-20) | 2 | 1 Pvt., 1 Sgt. |
Ages of 106 Draftees of the 16th Infantry Regiment, 1st Infantry Division, June 6, 1944:
| Birth Year (Age) | Men | Ranks |
|---|---|---|
| 1909 (34-35) | 2 | 1 Pvt., 1 Pfc. |
| 1910 (33-34) | 2 | 2 Pvt. |
| 1911 (32-33) | 4 | 1 Pvt., 1 Pfc., 2 Sgt. |
| 1912 (31-32) | 5 | 1 Pvt., 3 Pfc., 1 T/5 |
| 1913 (30-31) | 2 | 2 Pvt. |
| 1914 (29-30) | 5 | 2 Pvt., 2 Pfc., 1 Sgt. |
| 1915 (28-29) | 3 | 2 Sgt., 1 S/Sgt. |
| 1916 (27-28) | 6 | 1 Pvt., 5 Pfc., 1 S/Sgt. |
| 1917 (26-27) | 3 | 2 Pfc., 1 S/Sgt. |
| 1918 (25-26) | 4 | 1 Pvt., 2 Pfc., 1 T/4 |
| 1919 (24-25) | 4 | 3 Pfc., 1 Sgt. |
| 1920 (23-24) | 17 | 4 Pvt., 6 Pfc., 1 T/5, 1 T/4, 4 Sgt., 1 S/Sgt. |
| 1921 (22-23) | 6 | 6 Pfc. |
| 1922 (21-22) | 16 | 9 Pvt., 6 Pfc., 1 Sgt. |
| 1923 (20-21) | 5 | 4 Pvt., 1 Pfc. |
| 1924 (19-20) | 15 | 13 Pvt., 2 Pfc. |
| 1925 (18-19) | 7 | 7 Pvt. |