So we know Spongebob is older than 16 since he drives, but Squidward here has an mystery age.
He has a full time job just like Spongebob but he has driving skills better than Spongebob.
So he has to be 17-18. He's always been said to be "mature" based on the first episode.
He live alone like Spongebob, so he would have to be at least 18 but I'm guessing. Laws are different underwater.