How to deal with uncooperative employees who are dismissive of your authority because of your age?

I’m 24 and a bar/club manager and there’s a few older employees here who don’t take me seriously and refuse to listen to me because they think I’m just dumb because of my age. Or they make fun of the fact that I went to college and assume that I think I’m better than them because of it. I try to be a really nice and supportive boss and I’m rather assertive but it keeps failing. I don’t want to fire them as they are really good at what they do.