26 year old man dating 18 year old
Just know that it most likely won't be a long term relationship.
Oh they should be of legal age, so you're good there, but IMO it would probably be better if they were also no longer in high school. I'm 19 and I dated someone 11 years older than me.
Biggest thing is the maturity, most were completely obscure doing who knows what in their own world, NO THANK YOU!! My current girlfriend was 18 when we started dating (I was 25), and we've been together for 9 months now, she's 19, I'm 26 now.
The fact that she's actually smart and mature about things is what made it different from the other girls her age.
At 18, I was very mature for my age and I found dating someone older meant we were near enough on the same level. That being said, there's a good chance that an 18 year old isn't going to be too mature, and while it might not seem like a problem initially, it's probably going to start becoming more and more annoying the longer you spend with her.
Whilst I acknowledge that previously you've had issues with dating younger girls, you shouldn't paint everyone with the same brush. Compound her immaturity with having to hang out with her 18 year-old friends, and I could imagine it would be really difficult to deal with.