No this isn't bad. the age difference is no reason for them not to be dating. I don't understand the other answers who act like there is something wrong with this. I think 13 is a fine time to start dating and nothing is wrong with a 15 yr old liking a 13 yr old.
Raising kids into adulthood is what we aim for not suppressing them to stay children. It's important to teach them how to interact with members of the opposite sex while they're still in the parent's care.