is an 8 year gap too big?

I’m 23f, I’m talking to a guy who is 31, he’s hesitant about having anything between us. Is that age gap too big? anybody here have any personal experience with this? I like him, I’ve always been told that I’m “mature” for my age and I don’t think I’m wild or whatever. I guess I don’t understand why he would be hesitant. any advice/ opinions would be great, thanks in advance.