If they are delaying adulthood, then they should all delay voting as well. I would support a law that would prevent any person still being supported by a parent or by the government (as in on welfare, not with a government job) from voting. In the first case, they really have no skin in the game and will not suffer the consequences of their choices. In the second, they have an interest in keeping taxes high and government big to keep supporting them, but have no responsibility to contribute to that support.
That is basically the way our government was originally formed!
Only landowners were allowed to vote originally.
This in my opinion is THE biggest problem with our system today. No one that is taking ANY form of direct government assistance should be allowed to vote. No citizen should be allowed to vote themselves the treasury.