A pretty girl is a female that seems pleasing to the eye. It does not matter if they are 18 or 98. If they have something about their appearance that makes you want to observe in a pleasant manner, and they are female.. you have found a pretty girl. Or perhaps a statue of one.
However, do not be fooled by outward appearance. It does not reveal what is inside. For I have met people who looked breathtaking.. and then they said ugly things. They began to -look- ugly to me. And, I have met people who may not look so great on the outside, but they are kind and have a wonderful soul. It is -those- people who take my breath away.