I'm not saying that 16 yr olds should or shouldn't be able to vote. My only point is that when people say things like "16 yr olds are old enough to marry, serve in HM Forces....so they should be able to ????? as well....", it really isn't as simple as that.
As this article points out, to join the Army at 16 you need your parents permission, you won't be sent to a war zone until you are over 18, etc. In the same way you need parental consent in England and Wales to marry at 16, .... well the article explains it better than I can. The point is, there seems to be a transitional period between 16 and 18, where you can start to do certain things but often still have various forms of legal protection in the meantime. It's a grey area, a bit less black and white as some people commenting on here seem to imply.