Well, in earlier times, such practices, for example 18 year olds marrying 12/13 year olds, were fairly common. And from an evolutionary perspective, the moment animals have fully developed sexual organs they are capable of reproducing, the same could be said of humans. However, unlike animals, humans have an extra timespan after this time, where they reach emotional maturity at roughly age 18 - 21. Which is why an adult having a relationship with a teenager is generally considered unnaceptable or wrong in most cultures.
Nonetheless, as with all things there is a difference between having certain thoughts and acting on those thoughts, but there is often a very fine line between the two.
Saying that, I personally prefer girls who are slightly older, or my own age.