An expression used to describe someone who naturally looks quite young and around the age of someone who has just graduated from uni (university). They are however much older and in their 30's. Looks can certainly be deceiving!
Person 1: I'm just sharing some email links with you to assist. I assume you are new to the workplace.
Person 2: Ummm, I've worked here for many years now. I'm actually in my 30's.
Person 1: Woahhhhh! Sorry but you look like you're just out of uni! I cannot believe it!