I agree that Ernie and Bert are two guys in their early 20's (disregarding Ernie's remarks about having to finish grade and high school).
Oscar is definitely an adult.
Grover is a tough one. Being a literalist, I can't settle for the "technically ageless" thing, so I have to assign him something in my mind. I would say that he's around 8-10, and Cookie is about the same age, only with extreme ADD and OCD
. As for the job thing, I guess things are a little different with monsters--their mommies expect them to get jobs earlier than human moms do. So Grover became a waiter and Cookie hosted a TV show of classy drama.
Prarie Dawn being 6 or 7 fits. However, take all of this and imagine what one of her little pagaents would
literally be:
a little 7-year-old girl putting on a play, directing two weirdos in their early 20s (Ernie and Bert), and three hyper elementary school boys (Herry, Grover, and Cookie).
It gets quite weird. But then, it's a weird neighborhood, anyway. (Forever burned in my memory is Gina turning to the audience in exasperation over some silly thing that just happened and saying "Do you think this happens on other streets?")