I agree about the idea that age isn't really an issue for the Fraggles, at least not like it is with us, none of them really 'grow' up, that's part of the show's sweetness.
The show did state that Mokey was the oldest, as been said here, but never talked about age ratio with the others. Age doesn't seem to matter, but the overprotectiveness with Wembley really gives me the idea he's the youngest. Mokey is sweet with all four, but with Wembley (and Boober, awww) she's so loving. Wembley and Boober are close too, in a different way than Wembley and Gobo.
I think they do act like siblings sometimes, but I'm sold on the idea that Wembley is the youngest *Hugs little Wembley*