One might say that the writer's POTUS estimations are on the low side. The reason I say this is because he hasn't addressed those over 45 and the role they can play and it is a substantial role at that. And keep in mind what the life expectancy was back then. Someone 45 years old was typically considered to be an old man in colonial times!
Support roles. Military strategists know what this is. Drivers, medical personnel, logistics, supplies, reconnaissance and on and on. There are many ways those who are "over aged" could be called upon.
To discount those people and the vital role they could play creates two setbacks. One is that you give that large group of "old guys", who want to help, no expectation of participation when they could be used in a beneficial way. Never waste manpower. Molly Pitcher (the old guys) might even be needed to put her pitcher down, step in, man the guns and save the day.
The second is that limited forces are diverted in that the "young guys" would be forced to do the things the old guys could be doing. Instead of having your fighting men fighting you now have to expend that resource away from fighting thus weakening yourself in the process. Let your support people fill those roles and keep your fighting units strong.
Much like Leonidas said to Ephialtes in the movie 300...
From thigh to neck, Ephialtes.
Forward The Molly Pitcher Brigade!