Wiki says the ages are vague. "The age of the characters is unclear."
Bert isn't a bummer though according to the same page: some of Bert's interactions with female characters appear to show that he is attracted to women: serenading Connie Stevens in the Some Enchanted Evening segment of a first season episode of The Muppet Show and recording a song about his girlfriend, I Want to Hold Your Ear, which was released on several albums.
Ernie might be though.