I think the wide variety of ages is a good indicator that setting things at the age of majority is not sufficient...

16: drive
18: smoke, sign contracts
21: drink
25: house of reps
30: senator
35: president

There's a clear scaling up. I know that driving can effect other people, but the idea is that there is nothing about being 16 that makes you worse at driving than any other new. Which may be wrong. But at least in terms of skill it's a fairly simple matter of coordination and a few dozen hours of experience. For smoking we say 18 is the age at which you understand the health risks. For drinking, you have to understand the health risks and then the risks to others that come along with being drunk. House-->senate-->president is a clear scaling up of difficulty. So, don't we specifically tailor the age to whatever it is that's at stake and what qualifications are required? I think taking out a mortgage is clearly a simpler thing to understand than an entire economic policy (also there was more going on than just a failure of personal responsibilities...).

It seems fairly evident that the drafting age would have to be raised if the voting age was raised.