Theoretical explanations for the preference of human males for young females as mates include the facts that humans tend to form long-term mating partnerships, and that female fertility is limited by menopause and, therefore, age. The converse of such an explanation suggests that species that appear to lack long-term pair bonding and menopause (such as chimpanzees) should not exhibit such strong preferences by males for young females.
Additionally, the ridiculous emphasis that society places on 'youthful' looks also would play a part imho. For the record, I'm actually 19 and while I accept that a lot of older men will always prefer younger girls I still think it's somewhat gross because of the totally different mindsets that exist.
However, I have dated guys in their late 20s because I believed that they would be more mature compared to guys my age and ended up learning that age alone isn't an adequate indicator of maturity. So I do agree that a lot of young girls do date older guys because they believe that they would be more settled and mature.
At the end of the day, however, it's up to the two people involved and as long as they're happy, it's all good.