Ernie and Bert are really the only ones I can't associate any actual age with, mainly because they really don't fit any of the conventional age brackets: they play with toys and games and do recreational activities typical of little kids, however, they share an apartment, do their own laundry, make their own meals, Bert listens to the news, all of which adults do. It's especially vague back in the days of Jim and Frank, because they basically ad-libbed much of those old E&B inserts.