Someone in his 20s (or even 30s) that still acts like a 17 year-old. Overripe teenagers who are male often try to date teenage girls. Overripe teenagers who are female not only act like a 17 year-old, but dress like one, too.
Jim: What's up with Jeremy? He's 35, working at McDonald's and dating a 19 year old?
Eric: He's just being Jeremy.
Jim: I think he is an overripe teenager.