Age gap

So this guy and I have been "seeing" each other for almost a year now we tell each other that we're going out but I don't want to tell any of my friends. he's told his but it's easier for him because He's older and not in school. I'm still in school and there's been rumours going around about us

The BIG problem is the age gap he's 21 and I'm 15 and yes yes yes I know that sounds soooo bad but if I was 31 and he was 36 it wouldn't sound as bad, he's told me he'd wait until I make my mind up and we've been through so much together and he's helped me through a lot of shit to do with my dad he's the reason why I'm still in contact with my dad

I don't know what to do