First off, there are a few requirments for voting eligibility, and they differ by state. The most common answer is citizens under the age of 18. They don't really want kids who aren't in tune to what's going on in the country who will vote for someone who will potentially make a BIG impact in the nation. They also don't want convicted felons/criminals voting.
The specified requirements are as follows:
Voting eligibility:
You're NOT eligible to vote if:
Hope ths helps!