The "19" figure is mythical - just google "average age of soldiers in Vietnam" and you'll see that the real figures are known to be younger than other recent wars, just not so extreme - around 22-23 years old depending on the exact statistic you're working from.

That said, the Paul Hardcastle song "19" is still pretty good. [0]

[0] https://www.youtube.com/watch?v=b3LdMAqUMnM