Once we cross whatever arbitrary line is drawn for adulthood restrictions based on age start resembling rather closely restrictions based on ethnic heritage.
Except that we can very clearly say that people below a certain 'grey area' between about 16 and 21 (depending on who you ask) should not be trusted with the vote or indeed most privelages of adulthood - it's not just discrimination in the same way as discrimination based on race. I know a lot of people on here like to believe that very young men and women can comport themselves as adults, but that's simply not the case.