Why it's creepy for a 20 year old to want to talk about sex with them?

This isn't a situation where she's at risk or anything, it was just a throwaway comment about sex in a group chat by an older man in response to something she said, and my response that it was creepy, and her arguing with me over it. It's so frustrating to try to explain something to teenagers that are convinced that despite going through similar experiences themselves, people who aren't them (and are older than them, especially) can't possibly have any relevant advice for them. Especially frustrating because I know I would have argued as well when I was 16 and going through similar things, because I thought I was a unicorn that was just as mature as adults and that's why there was no problem. It took growing up a few years and seeing how the maturity levels of teens look from an adult perspective to see how creepy it actually is.