How is Spongebob?

He has to be at least 16, becuase he's in boating(driving) school, but then again, he graduated high school, so he has to be at least 17. But people call him a kid. Although, in "Bully" it's zei he's in a class for adults. And he acts really young, obsessing over toys and stuff. When youre 17, you're not really a kid, but you're not an adult till 18, aren't a "goofy goober", don't love toys (mostly), and so on. I'm Just curious.