Most people younger than 18 would vote the same as their parents. They don't have independent political leanings yet. And I can also imagine situations where domineering parents actually tell their children how to vote, and threaten with punishment if they vote otherwise. Allowing people under 18 to vote would be, in my opinion, and extremely stupid idea.
Personally I think the voting age ought to be raised again to 21, though the evidence supporting that opinion is purely anecdotal, so I am open to being convinced otherwise. I went through a lot of mental changes between the ages of 18 and 21, and looking back, I think I was extremely immature at 18 (not that I acted like a child, but that my opinions, especially political ones, were not well-thought-out).
Of course, the main argument for leaving the voting age at 18 is that 18-year-olds can be conscripted if a draft is passed by Congress. The idea that someone could be called to give their life for their country when they are not yet allowed to vote is really the ultimate form of "taxation without representation".