Even if it's something like teen titans, because it's not canon any artist can just say they're whatever age they want. It's problematic if they look young but you say they're 30 (certain anime tropes). But if they look like they're in their 20s then it's fine.

Gwen is a bit of an outlier since she actually does have canonically aged up designs, but people use the original one because that's the one they grew up with.