I used to think that. What happens between childhood and adulthood that makes us, well, ugly? I've heard it explained different ways.

First, children are attractive because they "need" adults to care for them. Most adults don't have that need. So in a sense, it's a survival tactic (and a darn good one).

Second, puberty changes the mind and the body. The world is still what it is, but children aren't capable of seeing all the "gray" issues of life until puberty. They aren't missing out IMO. That leads to the next point:

Third, I think many adults allow the desires and weight of the world to get to them. Money, power, greed, etc. There is something good and pure about childrens innocense (not saying they can't be little brats once in a while) that we forget about as adults. We could still have it, but it's rare.