For various reasons, the voting age is set for 18; no higher, no lower. It's basically a moral imperative that the age is no higher than 18, under the old-enough-to-fight, old-enough-to-vote rationale. Many believe that lowering the suffrage below 18 could not democratically happen for a laundry list of reasons. Parental coercion and influence is probably the biggest factor keeping mid-age teens from the polls. How will you ensure that the kids aren't getting paid off by Mom and Dad for adhering to conformist views?
One way we can view this influence in action is through all-school straw polls, most commonly held around Presidential election time. For the sake of this analysis, we will look at Middle and High School results (age ~11 to 18). We find that children of openly political parents are very likely to adhere to their parents' views. (This is moot if the two parents support opposing candidates!). However, this correlation is no causation.
From the sample of children of political parents, when these youngsters voted against their parents' candidate, more children of conservative parents voted for the progressive than children of liberal parents for the conservative. While youth are often by nature progressive, this natural tendency does not explain the full extent of voting trends in school straw polls.
Media geared to youth tend to be left-leaning; I call out Time's TFK publication in particular. Some networks take sides (Rupert Murdoch?), others inadvertently show support for one candidate over another, by amount of airtime and general portrayal of a candidate. If a majority of publications favor one party over another, who gets the benefit of publicity? Young people like a youthful leader, and, in recent elections, there has been no shortage of youthful Democratic candidates who have graced the covers of nationwide publications available in school libraries. For children of non-political parents, the Media seems to be the most important factor in influencing a child's opinion about a candidate.
We also have to watch out for teachers; in their course of affairs, their political views may come out in discussion of current affairs. Teachers need to make sure that their (younger) students receive a balance of political views, if such opinion does play out in the classroom.
Most importantly, though, young voters are most drawn to charismatic candidates- such as Barack Obama. In the Saint Anselm's 2008 Straw Poll, the current POTUS drew a majority of votes across all grade levels, especially in the younger grades.