For instance, the last US Constitutional amendment, passed in 1976, declared that voting age will be 18 in all states. Prior to that, some states made it 21 (and some states still sell alcohol, for instance, only to people over 21 years old). And minimum marriage age can be 16 or 17 in some places (in Israel, for instance, it's 17).
Why 18? Good question. We can be certain, however, that the fact that it is *precisely* the half of 36; *precisely* 3 times six and 2 times 9 - has nothing whatsoever to do with it.