I dated women in their 40s while I was early 20s, we got some funny looks occasionally but generally no-one gave a shit. What two consenting adults do is their own business. Though now I'm in my early 40s, I don't think I'd have a lot in common with someone at 21; and maintain that just because it's 'legal' doesn't mean it's ethical, someone at 18 just does not have the life experience nor fully-formed pre frontal cortex to deal with someone who's fully-adult. Ripe for abuse imo.