According to my own experience, most people add users based on their looks. People used to add me a lot when I had a personal photo. Thankfully now, no one adds me for my looks. They rarely add me actually. I always accept friendship requests (even when some of the profiles are creepy), and I never delete people in my friend's list. I rarely add anyone, even when I like their answers. I'm too shy to do so.
It is not possible to predict real age or look of a person on this page. I add somebody who has answered something similar to my thinking. If he or she is answering intelligently I pick up he or she to send a friend request. Unfortunately I cannot send friend request to anonymous users and some of them are really good.