Humans are weak to peer pressure pretty much forever. Kinda' just the card that's dealt by being social animals. More or less weak is a question, but considering who we let vote as is I don't think you can really argue it's something to considering unless you support significantly more stringent restrictions on voting.
Which, uh. Hasn't worked out particularly well, historically.
Humans don't finish developing mentally until they're 25-ish. Ability to comprehend choices and consequences is one of the last thing we gain. Letting 18-year-olds vote on things they aren't really able to understand the effects of because they're biologically incapable of that perspective is already a bit frivolous. Lowering the voting age to 16 would be irresponsible.
If it's irresponsible, so is allowing anyone over, what, fifties, sixties? To vote. As well as anyone with a psych condition that impairs thinking. Probably have to throw in poverty conditions, poor sleeping or eating habits, gods know what else. Lots of shit messes with our ability to comprehend choices and consequences pretty hard. If you're going to restrict access to the vote based on that, you're going to disenfranchise a pretty damn huge chunk of the population (and, probably more realistically, invite straight-to-hell levels of corruption from whichever group gets to decide where the lines are).