The human brain is not fully developed until age 25. That is when it should be legal to vote.
If our country can call on 18 year olds to fight in its defense, then 18 year olds should be able to vote. Besides, there are plenty of 25 year olds who don’t think any more rationally than a 2 year old, no matter how physically developed their brain is.
I’m not so sure about 25 anymore, based on what I see daily.