Speaking as someone who is between 16 and 18, I think it's not the worst idea I've ever heard (with half an eye on the national service plan), but I definitely can't support it. Personally, the ability to have a say in the democratic processes of the country is the best indicator of whether one should be considered an adult or not, and, on balance, I would rather keep the voting age at 18 than come of age at 16. And, simply, personally speaking, I feel like I don't yet have enough lived experience to cast a considered vote, nor do I feel I have the maturity (he says reading through his past Forum posts!)
I have heard it argued that it would do something to quell political apathy amongst younger people, but I'm not sure I quite see how 16 would be a magic number for that, either. Is there a particular reason why one might be more politically active during one's A-levels?