LEGO Star Wars age limits?
For a guy who's 20 you can ignore the age limits. LEGO used to label set with "N..99" age limits but these days that sort of humour is less appreciated. It's still true, though.

I suggest focus more on what he's interested in, and what you think he'd enjoy building. Within the LEGO Star Wars range there are quite a lot of options. If he's mostly into collecting toys I suggest starting small, but if he's into spending hours building stuff there are options that go right up to metre-long display models.

I think your main risk is social or financial. Even if you misjudge his interest, if he's into Star Wars he's likely to build it, display it, and just not ever get round to buying more. But especially the first time, don't spend more than you're willing to have him go "what? No" at, and maybe be careful about where he opens the gift. If he has judgey relatives who might say "hahaha kids toys grow up" or similar, try to avoid. It's all obvious, but many AFOLs have had experiences like that.

That said, I've given LEGO for work "secret santa" events as well as other random times, and it's usually gone down well.