I always assumed that Spongebob was anywhere between his early twenties and his early 40s. Squidward is anywhere between his early 30s and early 50s. Krabs is a war veteran, so maybe he's in his 60s or 70s, possibly older (but he's a feisty guy and doesn't necessarily ACT his age, nor does he want to retire.) Sandy could be anywhere between her mid twenties to mid thirties I guess.
I think what I assume more than the actual ages, are the age differences between the characters.
Spongebob seems to be the youngest of the cast. Patrick is around his age (as we've seen them as babies around the same time.) Patrick COULD be slightly older (though I'm only assuming that because of his lower voice and his height.) Sandy appears to be around the same age as them. (Though, given that most of the cast are sea animals that live very long lives, Sandy might be younger than them.)
Squidward is most likely older than Spongebob and Patrick by several years, and Krabs is definitely older than the main cast.
however, if it were revealed that Spongebob is actually older than Squidward, that would actually be pretty funny. Age is just a number, after all. lol