SpongeBob SquarePants was born on July the 14th, 1986.  This is confirmed in the first half of the 10th episode, of the 2nd series - "No Free Rides" - SpongeBob's D.O.B. is shown on his driver's license.
Squidward Tentacles was born on October the 9th, 1977. 
Patrick Star was born on February the 26th, 1986. 
Eugene Krabs was born on November the 30th, 1942. 
As for SpongeBob, I am 100% sure of his age and have an appropriate source. As for Squidward, Patrick, and Mr.Krabs, I got their D.O.B.'s off of a SpongeBob encyclopedia page. Though, I am fairly sure that Mr.Krabs's D.O.B. is mentioned in the special episode, from series 5 - "Friend Or Foe?" Thus, I have listed that as a source also.
I hope this has helped.
 Series 2, episode 10(a) - "No Free Rides"
 Special episode, from series 5 - "Friend Or Foe?"
· 10 years ago