Hitler's social order isn't the direct ancestor of our current social order.
British and Americans drafted at 18. British starting in WW1 as far as I can tell, Americans in WW2. The voting age in the United States was lowered for all states to 18 around the time of the Vietnam war (1971 to be exact), specifically on the notion it being unfair to draft 18 year old to fight in a war they couldn't vote on.
The US seems to have lowered it from 21 to 18 in 1942.
With googling I can't find easily when the US made 18 the year in which people can engage in contracts. But I think that's generally more central than voting and draft.
I would expect that age to be at 18 in the US before WWII.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.