18 is the correct age.
Children should not be allowed to vote and should not be allowed to do the things mentioned in SP's post.
It seems absurd to me that children may be allowed to vote but are not allowed to purchase alcohol, or cigarettes, or enter (with the exception of necessities) contracts. If children of 16 are allowed to vote, then surely they should also be allowed to go into a pub and buy a pint! If we are to trust them to have the decision making powers to cast a vote, why can't we trust them to buy a packet of fags?
I don't know about you, but I don't want to see 16 year old kids in pubs or puffing away on fags.
There are too many anomolies, and therefore it strikes me as sensible to keep the above restricted to those who have reached the majority, and to change the situations in SP's post to 18.
Plus, despite the out of hand dismissal by some, 16 and 17 year olds do tend to be more left leaning. I used to live in Toynbee land when I was in the Sixth form, and TTT is right, it was only when I grew up and had to face responsibilities and began to understand the ways of the world, did I realise my immature view was wrong.