Okay time for my two cents. I agree with alot of stuff that's been said, particularly from Danny and Tedwca.
First: Yes, in the middle ages life expectancy was low, around 30, so you wanted to have kids in your early teens. That said, as was brought up, the middle/dark ages are not a proper representation. There was widespread disease, malnutrition and poverty. Take the "great" civilizations of the past: Rome, China, Japan... all these cultures had life expectancies well past 60, into the 70's and 80's even. Look at Ancient Greece... all our good old philosopher friends lived well into their 70's or 80's, and that was a few thousand years ago.
Now, are we meant to live this long? I agree we are extending our lives far beyond what was biologically intended. And I agree we are as a result having kids later than we are biologically disposed to. Yes, if you are a woman and hit menopause you shouldn't be having kids. Yes, if you are a man, and at 50 you need cialis of viagra to get it up you also shouldn't be having kids. It's nature's way of saying your past your prime, and we should live with it.
Now, simply because we are biologically able to conceive at 14, I don't think we should. I don't think it's a question of morality, but of ability.
Let's take a step back and look at our primate cousins. Simply because they are at an age able to conceive, it doesn't mean they will. In fact, it will likely be many years after child bearing age that they gain the social opportunity to do so. Males shall have to ascend the social hierarchy to the point they gain access to females. By this point, they will have acquired the experience and knowledge necessary to raise a child by assisting others, and watching others do the same within the group. Even for females in the group, while their first child may come shortly after reaching estrus, it is likely that they shall have assistance in the child's raising from other females.
To conclude, I agree that we are able to have children at a very young age, and I also agree our culture tends to "extend" childhood because of longer lifespans. That said, I think now a days we also have kids far too late in life - far later than our biological clocks would like. Not to say we should be having them at 13, but definitely earlier. As for conceiving, I do not think morality has anything to do with it. I hate to say it, but arguing for sex at 13 to be morally wrong is an extremely ethnocentric argument. In developing nations, kids at 13 are far far more independent than those in North American society. It is different; not morally wrong. Lastly, I agree with those who have posted before that the issue with children at 13 is a question of experience and knowledge, and that it takes a few years past child bearing age for us to acquire the necessary skills. I believe the bottom line rests with the ability of the parent to care for their child, and I am positive there are 13 year olds out there who will do a far better job raising a child than some 30 year olds, 40 year olds, or 20 year olds, living next door.