One reason may be that persons younger than 18 will not always make responsible decisions, so entrusting them with electing officials might not be prudent.
A person younger than 18 might not vote for themselves, to reflect their own beliefs, but they might be influenced or even pushed to vote for a representative or measure of their parents' choosing.
People younger than 18 generally are not educated enough about politics and policies to make smart choices when it comes to voting.
