They're having fun. Their fans are having fun. They're still playing the music they want to, to good crowds. If they can still put a good show on, why shouldn't they keep playing. Rock and Roll is about maintaining a youthful perspective, and if they can still pull 18yrs olds, then more power to them.
Yeah, for that matter, anyone over 60 should immediatly stop living their dreams or pursuing them. It is time to give up on life and live your last 20 or 30 years or however long you have being a miserable, grumpy old man.
Black Sabbath released their first album in 1970. How old do you think their fans are?
And honestly, there is no age limit to good music.
Of course, if one is an old fart, devoid of any discernible musical talent, it does feel great being able to sit in front of a computer and talk **** about hugely talented and successful artists who've been releasing music since before The Beatles even got a record deal.