At my age (33) I can’t really see myself dating someone too young to legally drink in the US. Can’t really see myself seriously involved with someone over about 40. Call it 10 years either way for me.
As to underage, I would have issue with someone being over 18 being involved with someone under 18. Once they hit the age of majority they can do what they like.
My husband was 27 when he started dating me. I was 18.
Now* I’m* 27. I rarely think of our age difference, it just hasn’t been an issue. The only time it really crops up is when we start discussing childhood TV in syndication. Or when he says something about watching Star Wars in the movie theater. Then I tease him for being older than dirt.
I’m 41; if an 18-year-old developed a crush on me (which would undoubtedly require divine intervention) I’d jump into the relationship with both feet!
After getting down on my knees and thanking Aphrodite, of course. 
When I was 21 I was going out with a 36 year old - worked out fine until we broke up, and that wasn’t due to age or anything related to it.
I agree that once someone is over 18 it really is up to them what they do and who they date, although I tend not to have that much in common with people under 20 as I find their outlooks too juvenile (understandably).