Mine definitely are the "attractive 30-something" subtype rather than completely average people, but I figured I'd show my main duo!
He's more late thirties and she's only approaching mid thirties.
Overall I think keeping the proportion of the face, even in toonier styles, is important. But part of the reason that adding wrinkles makes them look super old is because wrinkles on a real face are a lot more subtle, and honestly I only start seeing deep/significant wrinkles in people older than 40. That's largely why the male character, Nox, only has a couple lines under his eyes and deeper dimples. I wouldn't start adding things like crows feet or forehead wrinkles until he was well into his 40s and 50s.
Another way to show age for characters in their 30s is to draw wrinkles while they're expressing an intense emotion that makes their face scrunch up, but wouldn't be noticeable otherwise.
Honestly though, it's not only about how they look in the face, but how they present themselves, their attitude, and even their burdens. If they act like a 30-something would, have problems associated with that age range, or have a lot of responsibility on their shoulders, it's easier for the audience to buy that the character is older. It's soft traits like that that influence how we guess age in real life, too. (and why people often assume I'm in my 30s when I'm still in my 20s "orz)
