I agree with you 100% PeterCH, at 13/14/15, perhaps even older, kids just don't have the maturity to be making such big, possibly life altering, decisions.
Nor do I believe that a girl is entitled to making these decisions sooner than a boy.
Trouble is, where do you draw the line? As you said, it boils down to each individual and their up-bringing, morals, values, and their opinions of themselves, etc...
I know people, well into their 20's who haven't had sex - because they haven't found someone they feel comfortable doing it with, because they want to be married first, a whole host of reasons... But at the same token, I was in primary school in 1994 and I knew a 12yr old who was pregnant.
I guess in the end, it's up to the parent's to ensure that they instil the right values in their children