A cute nickname for someone you have affection for. Normally this nickname is given by an older guy to a younger girl, of which he either is with or fancies. However, the use of this normally leads to the girl thinking of the guy as a big brother, rather than dating material.
"Hiya Pup!" Man, she looks good
"Hey. I just got a new boyfriend, I think he's wonderfullllllllll. I want you to meet him cause you're so special to me."
"...Sure, I'd love too." Stupid Nickname
n. A younger man that an older woman is involved with intellectually, emotionally, romantically, and/or sexually. Men are often referred to as "dogs", hence "pup" for a younger man. It is commonly believed that a pup can be "trained", i.e. the pup will easily learn and do what is necessary to keep the woman happy with their relationship. While the pup provides energy and enthusiasm, the woman keeps the pup continually challenged with her knowledge and experience, as well as keeping him feeling safe and doted on. They provide good companionship for each other characterized by fun and affection, not necessarily (but often) physical and/or sexual. Can be used as a verb. Related entries: papertraining, chewed slippers, Love at Goon Park.
Just because she looks good for her age doesn't explain how many pups follow her around.