The Mayhem:
Floyd & Janice are at least mid-thirties. Dr. Teeth is 42 just because, and Zoot is in his fifties. I have no clue how old animal is.
Kermit is 28. Still young but old enough to run the muppet company. But young enough to still be green, no pun intended.
Piggy seems to have aged. In TMS I'd say she's about 19 when she started. But now, she seems as if she's older, about 30.
Fozzie: 32. Starting to realise he's not going anywhere in life and so of course becomes a really bad stand up comic. Hee hee.
Gonzo? Probably about 24.
Scooter is 16.
Pepe and Rizzo are 21 as I believe prawnie stated, which makes complete sense.
Rowlf is around 50. He seems older and wiser, like he's been through the mill. ^_^
Statler and Waldorf are 92? Nice round number. They're older than 75.
Sam is not quite 40.
And, I can't place Bunsen and Beaker.
edit: I said Kermit's about 28. I think he's like Piggy, he ages a few years. Presently I'd say he's very early thirties. For some reason.