I remember a much earlier thread talking about how Spongebob exhibits both the qualities of children and adults. I mean, he sure acts like a kid, but then again he lives on his own and holds down his own job. I think that's what the SB producers are purposely doing: putting Spongebob in sort of this "netherworld" in between childhood and adulthood.
How do we know Patrick is 23? (Just wondering.) We know from "The Bully" that he went to junior college, so that should suggest something--and it's not too likely that he skipped any grades, if you know what I mean!
You can also see from the episode "Sleepy Time" that Mr. Krabs was born in the 1940s (can't remember the exact year.) So you could say he's meant to be a character in his late 50s/early 60s.