In the first Spongebob movie, Spongebob mentions that he has won employee of the month 374 consecutive times. That's 31 years and 2 months without any gaps. There is another episode where Spongebob and Squidward compete for an employee of the month award, and Spongebob mentions that he's won the award 26 times in a row, adding on another 2 years and 2 months. The episode ends with neither of them winning it, ending Spongebob's 2 year streak. Let's assume that he started another streak immediately after this and never stopped. Spongebob has to be at the very least 18 when he started working at the Krusty Krab, since he legally owns a house when he gets the job in the first episode. With that said, let's do some math.
18 Years + 2 Years + 31 Years = 51 Years
This makes Spongebob 51 years old during the events of the first Spongebob movie. So when Mr. Krabs calls him a kid, keep in mind that Spongebob is an old man who has been working the same minimum wage job non-stop for at least 33 years straight.