Muppets are ageless. They do not conform to the normal laws that govern we humans.
Big Bird, for example, is six. He has been now for 25 years. Grover, despite having a part-time job as a waiter, has a mental age of about 8.
There was a Bert & Ernie sketch once where Ernie pretended to be a Doctor when Bert was ill once, and the punchline was that after the examination, Bert asked for his diagnosis, and Ernie proceeded to describe why he was unqualified to reliably diagnose anything. He began with “Well, first I’ll need six years of Grade School, 7 years of High School, 4 years of College, 8 years of Medical School…”
Anyway, to me it implied that despite their independence, and Odd Couple living arrangements, they both had never been to school.
Jim Henson was too cool…
“Well, roll me in eggs and flour and bake me for forty minutes!”
The Legend Of PigeonMan