Why? If you say something with this conviction you also have to explain why you think it's 18 and not 17 or 16.why stop at 16? Why not give vote to 12, 13, 14 and 15 year olds? A line has to be drawn somewhere and 18 seems as good a place as any. ALthough there may well be some 16 yo who are intelligent eniough to decide who to vote for, most are not and don't have a clue.
This "you have to draw a line somewhere" sounds a lot like "we need to get brexit done". Doesn't mean anything, really.