Your theory just brings up unanswered questions.

Then it brings up this quandry; pbl mentioned how they could age by their insides just getting older. If Herriman was created old, then wouldn't his parts be SUPER old by now? And if so why don't we hear him complaining about a trick back, or something along those lines? Yet still the elderly rabbit can hop pretty darn fast (Land of the Flea) and survive a near fatal tumble down a whole flight of stairs (Crime after Crime, Bus the Two of Us) and even a nasty body cast (Bloo's the Boss). I know I'd be kicking up a stink if I were that age doing what he does half the time.

His appearance was old; his parts, however, were new. One could build a car from parts available today in the form of a 1908 Pierce-Arrow; it looks ancient, but in reality it's brand new.

Imaginaries ain't cars! :P

lol this is getting silly now.

But by now they would be old, too. I mean, Madame Foster's like eighty years old.

In the movie "Drop Dead Fred", Drop Dead Fred was the IF of the Lead character, both at her childhood and then 20 years later when he came back. He was virtually unchanged. he did not age in the same way that people age. (I do not think this was mentioned yet but I could be wrong)

IN my opinion, I think Imaginary Friends age Like Cartoons.

Cartoons do not age, some examples:
Tom and Jerry, Scooby Doo, Muppets, Buggs and Daffy, ect...
all these characters have been around for over 30 years and each of them have had new shows/movies in the past few years. The main characters never changed.

Basically they do not age, but they do develop over time. a good example of developing over time, is the Simpsons. Look at some of there Original 5min skits and they were extreamly different. thus over time, a cartoon character ages by becoming better developed. so IF develop over time, by experiencing new things, but not by getting elderly, ackes or pains.

I wish I could spell. :(
Monty :-/

Some of the Scooby-Doo movies did feature somewhat older versions of the characters. Daphne looked phenomenal. :)

Oh yeah? I'm pretty sure there would be some Imaginary cars out there in the Foster's universe! Huh, what an insult to them!
LOL8D j/k

I don't think that IFs can live forever, so somewhere along the line, they have to grow "old" and pass on. Then again, after seeing Mr. Pockets and how many generations HE'S been with...whose to say for sure? I guess that's one mystery Craig has yet to reveal to us.

Who knows, really. But I know in a few doodles I've drawn retirement-home age Eduardo, Wilt 'n Coco. They're funny when they're old.

Eduardo: Es you taken your tablets?
Wilt: *straining to hear* Sorry..?
Eduardo: ES YOU TAKE YOUR TABLETS?
Wilt: I'm sorry, but why are you yelling...?

*snorts a giggle* Still makes me laugh even if it'll never happen.

LOL 8D That's perfect! I hope you share those "doodles'!
And wouldn't that just make a grand fanfic! XD