We have no idea how old he is

The problem with this is that the movie (and the tv show) never exactly specifies that Spongebob is a child. Mr. Krabs declines to promote him to manager because he is not mature enough. The movie does feature a subplot where the Spongebob and Patrick "become men," but again, the movie does not specify Spongebob's age - it appears to be more of a question of attitude and maturity, not chronological age.\r\rAnd anyway, Spongebob is a SPONGE. How do we know that sponge childhood DOESN'T last 47 years? (Incidentally, I looked this up, at the risk of seriously overthinking this question, and discovered that the Antarctic lollypop sponge is one of the shortest-lived sea sponges, with an average age of 150 years. The rossellid sponges live for more than 300 years. And the Cinachyra antarctica is believed to live more than 1,500 years. So Spongebob could be considerably older than 47 and still be a mere child in sponge terms).

Not necessary to explain age.

The writers of the show take a lot of liberties in the undersea world of Spongebob. I think the fact that he's been the employee of the month for 374 months is used to establish the fact that he's the most obvious choice for the position of manager of the new Krusty Krab 2 and nothing more. The writers were just trying to build Spongebob up as the obvious choice and set him up for a big let-down, which would lead to him having to go and get the crown to prove that he's not a kid. This is further established by Mr. Krabs saying something like, "He's someone we all know and love. His name starts with an S." I think this is a case where we should give the writers a break, accept it that it's part of the plot build-up, and consider many of the obvious inconsistancies to be intentional parts of the story.