Here's an update...
From what I've read from an official source (the Sesame Street Unpaved book)...
Every single Muppet on the show is technically ageless. They are, however, written to represent certain age groups.
Big Bird is written to represent the age of 6.
Snuffy is written to represent the age of 4.
Grover is written to represent the age of 4.
Count is written to represent the age of 1,832,652
Prairie Dawn is written to represent the age of 7.
Elmo is written to represent the age of three-and-a-half.
Zoe is written to represent the age of 3.
Herry Monster is written to represent the age of 6.
So apparently, Ernie and Bert really are ageless, as are Guy Smiley, Telly, Kermit the Frog, and Oscar the Grouch,