I'll throw my hat into the bucket and answer this one. Actually, I just borrowed one of Gobo's nifty hats.
/hehe
Youngest to Oldest... Wembley, Red, Gobo, Boober, Mokey
Yes, youngest hands down is Wembley. Yes, Wembley and Gobo are friends, but he looks up to Gobo like an older brother, to me. And as someone else did point out, he was the last one of the five to take on the responsibility of a job. And when he's around Red, he's like that little kid brother who annoys an older sister at times, although he doesn't mean to do it.
Speaking of which, I have often felt Red was next in line, due to her still being childlike, playful, and stubbornness*(like a teenager) to obey the rules. Plus she all the time has to prove she's better than Gobo and that she can keep up with the slightly older guy. *I do recall, Gobo jokingly told us, "Remember, Red is not stubborn."
So now, it's Gobo, (who use to look younger than all the rest in the first 12 episodes, like a kid almost, but they fixed) I believe is next in line for age. Got the potential of being a good leader, but still being young causes him to make some no-so-good decisions and he's always in competition with Red, just because I believe, he's older than her and wants to prove why he should be leader.
Next, I'll throw in Boober, he may be small, but his experience on handling health emergency problems, knowing many home remedies, getting frustrated with the younger crowd, and sort of being like a father figure to the three fraggles(Wembley, Red, Gobo), warning them of the dangers all the time, to which they'd ignore a lot.
The oldest by far is Mokey. She's mentioned as being like a mother figure to the other four fraggles whom are all her best friends. So when Mokey & Red get a place together, I'm sure Mokey is delighted to have like a younger sister to share things with, even if she is messy.