The best age for a female to acquire a boyfriend is when she's old enough to be responsible for herself with out any adult supervision. I would say around 16-18 but that all depends on how fast she matures not physically but psychologically.
15-18 it honestly depends on whether you can handle it or not. When you're 13 you really can't go out with anyone. You need to be able to be alone with the guy at a place other then your house to actually date someone and have a boyfriend.