I would think that it is less common than say a 17 year old boy dating a 15 year old girl. At the same age, boys are less mature than girls, which would be why it is more common for a girl to date an older boy. But 17 and 15 aren’t too bad, I guess, if you really like him. It’s just up to you to put up with his immaturity, you can’t expect him to be as responsible as an older guy.