Age difference real opinions

Okay so I know these post always just get 'whatever makes you happy' as a response but I want to know what people's actual opinions are on this topic. I was hit on by someone 18 years older than me last (I'm 20 and they're 38) and although I wasn't interested it got me thinking... what's the largest age difference between that you think is okay before it starts getting abit 'weird'? (Assuming they're both adults and that it just stays as an opinion, not making anyone feel bad)