you remember wrong, the show started in may of 1999 making him 12 when the show started. (he was born on July 14 1986) When I posted this pic in on July 14 2012 he was 26 years old. Also the show would have been 13 years old, so their is no way SpongeBob could have been 2 years old when he got his job.
Actually his age was never said anywhere in the show but his drivers licence has been seen multiple times on the show. that where I got his birth date from and with basic math you can figure it out.
They probably goofed, then, or else weren't paying attention (or thought anyone else would) when they made it. I don't see him being 12 at the start, since, as I said, he had a job and lived on his own.