The point of Franchise isn't to assign maturity, But to select for candidacy or voting the most worthy to make the decisions. So for the vote, Your being asked to choose between politicians X and Y who have plans X and Y. I'd prefer people have some time out of Highschool before being given that level of important decision-making. At least.
The system doesn't exist to represent anyone, Except in civics classes, It exists to discern problems in the society. If the young see an issue, They can take it to their parents or older peers.
By 25? They shouldn't be living with their parents by then, So we need their suffrage by then.
Doing so will influence raising other age laws. For example, 18 year olds can still be taken advantage of for sex, Even if they can legally consent to it. 18 year olds should not be tried as adults for crimes, They are still teenagers. Again their brains are still developing. Setting 18 as the age of majority is not based on science.
Taking away the right to vote for 18-20 year olds is a huge mistake, As it leaves a voice out of those who need a voice. On the matter of sex, All I can say is that by 18 you should have an idea of when someone is taking advantage of you, I'm sorry if you're not that bright, The rest of society shouldn't be hindered.