Are 20-Year-Olds Not Able to Vote?

I was reading a letter that I wrote, a friend I'd put on my Hawaii trip this year and he was a friend from college. He must have given it to me years later because it was so funny. The thought crossed my mind, 20 year olds cannot be allowed to vote. And they shouldn't be allowed to sign contracts or make these decisions. You know, because I cannot think of a single decision I would have come to at the age of 19. But I'd want to be bound to now. This is really something that she must say applies not exclusively, but overwhelmingly to American kids. She thinks other children in other countries mature a lot faster.