What's wrong that? When you have a crush on someone it really doesn't matter what age he or she. There's lots of people who have a crush on a older man or woman and a young man or woman. So there's nothing wrong if you have a crush who is younger
It's natural that u have a crush on someone younger it doesn't really matter what age he or she is just tell him if ur ready maybe honesty will make him have a crush on u too