It’s kind of a sliding scale. Add 10 or 20 years to both ages, and the gap becomes less of A Thing. As you both mature (yes, you keep maturing into and past your 30s), the gap between “adolescent” and “adult” gets smaller and smaller. It can actually happen pretty quickly depending on how “worldly” the people in question are. No offense but while the law might consider the 18-yo an adult, that’s still a kid. And before anyone asks, I wouldn’t have too many issues with a 20-25 yo person looking to date an 18-yo. Not zero issues, but waaay fewer than this situation.

OP, if you’re just having fun go for it; but look out for yourself, and listen to words of caution. Plenty of 18-yo college freshmen have dated professors and come out just fine. At some point, the age gap will probably play a part in ending it, at least this time. There’s nothing saying you two don’t remain friends and return to a second relationship after you’ve experienced your wild 20s!