> I can't understand any other context for drafting 18 year olds other than taking advantage of their youth and incomplete development
Physical fitness surely? Isn't 18 or 19 around the physical fitness peak? I'm guessing that people are on average are significantly stronger and more robust at 18 than they are at 28. I think the British Army has 26 as the limit for joining in the first place for officers at least.
Also people are married by 28. At 18 they're generally single and more able to deploy.