How old is it normal for a 21-year-old girl to date?

My reaction is that it shouldn't matter, I have been with older guys and younger guys and its the person, not the number. But in this case I wonder. I am 21, and recently have been hooking up with a guy in his early 40s. It's a very casual thing (obv he is too old to bring home to my parents lol, not that I am looking for that at all) but it is nice to be with someone more mature and relaxed. Plus the sex is actually really really good. But he's at least 20 years older than me. Should that matter? I don't think so, but it seems unusual.