is age really just a number?

I've been with this guy Brandon for 2 years and 3 months. he has did me so wrong in the past because he cheated on me with three girls more than 5 times! i'm still with him but i don't love him like i use to. i got love for him but i'm not in love with him. he's 18 years old and i'm 17. there's this guy name Joseph that really likes me like seriously. he told me how he felt and i told him that i felt the same way. i would talk to him and break up with brandon but he's only 15! my friends said that if he treats me better than brandon does then go for it but i can't. i don't want to be a pervert! i mean.. he acts older than brandon does, takes things more seriously than brandon does but it's his age. would it be okay for my to give him a try or should i just let that go? is age really just a number or just a cover up?