i have always wondered because he seems like a kid put he has a job and a boating licence.
Answers (2)
In one episode Spongebob dreams of getting his driving licence. The card has his date of birth on it, along with other details, and despite the fact he acts far younger than his age, he is in fact 21. You can also tell he is not a kid by the fact that he shaves (only shown in one or two episodes).