3 years? that's a SMALL window, imho.
kg, my 15-20 thing was basically me saying that's the absolute maximum that i can swallow and still remotely believe there's a meaningful relationship there...
there exist good-looking people around that 60-years old range, (Liam Neeson is 60...here's a pic of Harrison Ford 10 years ago when he was 60:

)
and I'm sure there are good-looking dudes and dudettes around that 60-year range who AREN'T celebs (i just can't google them), so I can believe there's a meaningful give-and-take there...
the older I get, the more I find myself able to identify with older age groups...one of my wife's best friends that she works with is 53 (for reference, my wife and I are 26 and 27), so I can buy an actual emotional relationship in that type of gap.
it's not an EASY pill to swallow, but it's one I can buy...
but just because 18 is the legal age of consent (or younger in some states) doesn't mean the person in question is an ADULT....when I think back to myself at 18 years old, I was a freaking trainwreck of immaturity. I'm not the most grown-up person now (and i hope i never am....piss on that noise, i want to be able to have the occasional carefree moment), but my goodness, an 18-year old kid has never even had a REAL job or any REAL responsibilities...if you're older than your mid-20s, stay the hell away from teenagers.