From Bert and Ernie's wiki page

[froodle]

In the 1991 pilot episode of Eerie, Indiana, Marshal and Simon are subtly asked for help by a pair of twin brothers named Bertram and Ernest(called Bert and Ernie for short), because their mother has forced them to sleep every night in her 'Forever Ware' containers, thus having kept them at age 12 for over 30 years. This could allude to the fact Bert and Ernie have not aged in all the years that Sesame Street has aired.