One of the reasons boys that age are not now accepted in the USA/Aus/UK army is that in WWI, when they were allowed to sneak in, it was found that they weren’t suitable for trench warfare. At that age, it’s difficult to get them to stay awake while on guard duty.
My father cheated on his height (he was too tall) to maintain eligibility in WWII. (Obviously, different rules for different sections). He also modelled the “tall” end of a landing craft – even then you would think that would be referenced to a standard, but for whatever reason they used him. Which leads to the idea that any individual taller than him would have had difficulty fitting into the landing craft, giving reason for rejecting fit individuals just based on height.
At the other end of his life, he had to loose weight to qualify for surgery – he was too heavy for hospital standard procedures.
I suspect that it varied – there must have been some nasty little sods who were unusually hardy re teenage sleep requirements etc., and who made wonderful fighters; but on the whole, “sense is made”.
I recall from a novel with an early-World War I setting – a lieutenant, professional in the British Army, keeps finding ever-younger lads in his unit, who have lied about their ages to join up. He gets increasingly worried about the phenomenon, and increasingly concerned that it must be dealt with. Finally, by chance he runs into a cousin of his who has enlisted, and who he knows is aged fourteen. The lieutenant explodes: “JESUS CHRIST ! Now they’re sending me bloody CHILDREN !”
The cousin expostulates, “Jack, I’m not a child ! That’s beastly unfair !”
Lieutenant: “Oh, fuck it, I give up. Soldier, go back to your squad, before you’re missed.”
Either a hillbilly or a Texan. Rarely both. You needed a Jew, someone nicknamed “Ski”, and someone with an Irish surname. The guy from Brooklyn was often Italian. And some WASPs too. Maybe one from Boston who didn’t have a MA accent and smoked a pipe, and some midwesterners. There wasn’t much difference between Hollywood WWII infantry squads and B-17 crews.
I remember reading a WWII soldier’s memoir in which he wrote that his recruiter kept a drawerful of bananas, pastry etc. in his desk to fatten up kids who were right on the line of being underweight.