Yes, good points.
Obviously, the ones that are clearly children only would be Elmo, Zoe, Big Bird, Baby Natasha, Prairie Dawn, Baby Bear, Betty Lou, Roosevelt Franklin, Alice, Kingston Livingston III, Little Bird, and probably Rosita.
The ones that are clearly adults would be The Count, Hoots, Biff and Sully, Herbert Birdsfoot, Mumford, Kermit, Guy Smiley, Oscar, Benny Rabbit, Sherlock Hemlock, Bruno, Forgetful Jones, Grungetta, Sonny Friendly, Lefty, Fat Blue, Prof. Hastings, Gladys, Don Music, Simon Soundman, Sherry Netherland, Humphrey and Ingrid.
The rest of the main Muppets are just sort of floating in between: Ernie, Bert, Telly, Cookie Monster, Grover, Herry, Two-Headed Monster, Snuffy, Slimey, Davey and Joey Monkey.