I'd say he's about 18-25 years old cuz he drives, works, and has his own house. But maybe the laws are diffrent down there and he can do all that at 15 years old or something!!
Spongebob makes it quite complicated for everyone to actually know how old he really is. He acts like a little kid, but i would say he is at least 17 or older since he's driving. (well trying to at least) whee