How old is Spongebob Squarepants?

"Although his drivers license says he's born in 1986, SpongeBob creator, Stephen Hillenburg explains in an interview that he is fifty in "sponge years" as a joke. He explains that SpongeBob actually has no specific age, but that he is old enough to be on his own and still be going to boating school."

https://en.wikipedia.org/wiki/SpongeBob_SquarePants_(character)

According to SpongeBob's driver's license, he was born July 14, 1986. But he was already living on his own—and trying to get a job at the Krusty Krab—when the SpongeBob show debuted in 1999. In a little over a month, he will be 22 years old.

http://www.factmonster.com/spot/spongebobfacts.html

Okay, here's a little more information:

Creator Stephen Hillenburg has always said that he wanted SpongeBob to remain ageless, but the show has shown varying information about SpongeBob's age. In the season one episode "Sleepy Time", aired in January 2000, SpongeBob has a dream in which he receives his driver's license. This shows a birth date of July 14, 1986. Hillenburg stated later in an interview that it was only in a dream and cannot be confirmed. The same birthdate was seen in his temporary driver's license in the episode "No Free Rides". He has also taken a driving test more than one million times. SpongeBob mentioned in the episode "The Chaperone" that he could not get a date to his junior prom. In the movie, SpongeBob has 374 'Best Employee Awards' from the Krusty Krab on his wall. In the episode "Party Pooper Pants," one of Spongebob's boating school classmates is shown to be married and owns a house; SpongeBob lives by himself. Whether SpongeBob is a kid or not was also a very important part of the SpongeBob SquarePants Movie's plot. In the film commentary on DVD, it said he's an adult that acts like a kid.

http://cartoons.wikia.com/wiki/SpongeBob_SquarePants_(Character)

He is 23, 50 or has no age. Pick one.

"Although SpongeBob's driver's license says his birthdate is July 14, 1986, which would make the character 13 years old at the time of the series' "official" premiere on July 17, 1999, Hillenburg (creator) joked that he is fifty in "sponge years". He explained that SpongeBob actually has no specific age, but that he is old enough to be on his own and still be going to boating school."

https://en.wikipedia.org/wiki/SpongeBob_SquarePants_%28character%29#Development