Well Spongebob must be more than 16 since he has applied to a job at the Krusty Krab. He seems to be a bit of a kid (ice cream, jellyfishing, blowing bubbles etc.) so he's a frat boy (an immature young adult). His driver's license says that his birth year is in 1986 and the current year is 2006 which means that he's 20 now.
So 20-27 FTW!