didn't see the confirmation that they're children in the wiki, can you post where you found that? it would be unusual for children to live alone in an apartment together (see "In Popular Media" of the wiki page)
also, the later quote from the creator:
A last thought: If Jim and I had created Bert and Ernie as gay characters they would be inauthentic, coming from two straight men. However, I have now learned that many view them as representative of a loving gay relationship. And that's pretty wonderful. Thanks for helping me understand.