Scarleteen

Okay, so another issue that I haven't been feeling all too great about is seeking casual sex partners and not knowing their age.
I know late teens and early 20s can be a bit odd for everyone but it's definitely feels messy when you're going out hoping to have casual sex and finding out someone is
a lot older than you.

I get nervous around people my age and I want to approach people even if they may be 22 or older but I get nervous.
I also fear being or looking too younger for that person. How would I speak to someone who's a bit older than me without it being super odd?

I don't think you can control whether any one person finds it odd that you're talking to them as a potential casual sex partner, since people have a pretty wide range of views on dating people who are notably older or younger than they are and you probably won't know a particular person's feelings on the matter before you ask. It might help to get that question out of the way first thing, by opening with something like "I don't know if you're open to partners who are younger than you, but I wanted to let you know that I'm interested" so that you're acknowledging the issue. It sounds like you aren't super comfortable not knowing someone's age beforehand, and there's nothing wrong with asking someone how old they are when you're establishing if there's mutual interest or not.

I think overall, most people are pretty kind, if sometimes a bit awkward, in turning down people they aren't interested in dating or having sex with, for whatever reason; I don't think there's necessarily going to be a huge difference in how people respond if your age is a specific reason they aren't interested.