Drafting 18-year-olds: well, they are less likely to have employment in strategically important industries, they are less likely to be supporting a family, and they are seldom in mid-college, i.e. to be held aside for a commission upon graduation. Those, that I can think of, were the three big sources of deferment in the WW II draft.

They are also approaching peak physical fitness, both as to capacity for difficult physical work, and to the ability to recover from severe trauma.

[Edit: one should probably not exaggerate the idealism of draftees. Maybe it was there in WW I, but the accounts of WW II and since suggest that it fell off.]