This oft-repeated fact comes from the 1985 Paul Hardcastle song "19.", in which he states that the average age of the American combat soldier in Vietnam was 19.

I made a tangentially related post some time ago but I'll repost it below. The average age of those men that died in Vietnam of all causes was 23.1; the most common age at death was 20, with 21 being second, and 19 third.

Making the assumption that the "average" U.S. soldier in Vietnam was 20 or some other age based only on casualty statistics assumes that the distribution of casualties by age group was even, or accurately reflected the number of men (or proportion) of that age group actually deployed to Vietnam. It cannot be determined just on casualty statistics alone, without having the number of men of each age group deployed, that numbers of men in a certain age group were actually in inordinately more danger than others. A definitive catch-all statement cannot be made without further research, such as through combat unit records to determine the ages of each man that served, was killed, or became some type of casualty during actual combat with the enemy.

Some reasonable assumptions can be made, however: men of a lower rank were necessarily younger; officers were older; fewer older men were deployed as a portion of the whole; men serving in certain roles were by and large older.

Age at Time of Death	Casualties
17	12
18	3,103
19	8,283
20	14,095
21	9,705
22	4,798
23	3,495
24	2,650
25	2,018
26	1,414
27	917
28	768
29	710
30-39	4,927
40-49	1,156
50-59	121
60-62	4
Unknown or Not Reported	17
TOTAL:	58,193