I think of Scooter as being quite a bit younger than Kermit and most of the other "main" Muppets. I see him as a bright eyed kid - probably either still in high school or having just graduated recently. In that since, I think it's impossible for everyone to have grown up together. His introduction on the Muppet Show is probably the closest thing to being canon for his introduction, but I agree. There really isn't one Muppet canon.
Well, here's something else to consider. As of the Nancy Walker episode, Scooter's still in school. He even says so.
Check out this video (a little after the five minute mark) and Scooter talks about selling magazine subscriptions for school.
http://www.youtube.com/watch?v=ZChE15PN0mY
As for the Muppet babies thing...uh, I dunno. It's kinda odd. yeah, we could say that it's all part of Piggy's imagination, but there's the evidence on film in MFC. (Deleted video scenes or no deleted video scenes, some of us have the original airing on tape)
I think...just tossing an idea out there, I wish there could be a coffee table book called "The History of the Muppets". Collaborations could be from writers, Muppeteers, the Muppet Wiki, etc. to tell the once-and-for-all-truth about where the characters themselves grew up, whether the Muppet Babies play a part (or not) and how they met (or how some of them met as babies and then met up later).
Just how did Kermit find them all to hire at the theater anyway? We know about Scooter's connection with the theater. Oddly enough, there's a
detailed biography about Bo in a Muppet book from the late 70's/early 80's (including a detailed description about how he got hired). But...very little else is known about how Kermit hired the others. I could easily see Rowlf giving Kermit a hand (in Muppet history, they've been working together since at least 1969).
There's a lot to think about here.
Convincing John