If you think about it, you can consider Gonzo in season one as a kid, heck he even spoke like a baby in one episode, and was obsessed with a teddy in another. Season two, Gonzo became more 'mature' how ever lightly, still had that scratchy voice, and still obsessed with being the star. Season Three and the Muppet Movie, Gonzo finally got himself a job, a drivers license, and a girlfriend, like all normal teenagers do. Season Four and Five, Gonzo was much more mature, even going so far as to calling puppet shows 'sick.'
Then, Gonzo was taking Camilla on dates, even prize shows, in the JHH. Muppets Tonight, he tried to fill the void he left with the weirdest acts he could find...
So yeah, Gonzo is the only muppet who aged so much.