On the subject though, I dont think any artist should be forced to retire, thats crazy but you would like to think they show growth in their work like most musicians and keep putting out new music to reflect that... Nas and Jay being a prime example of recent growth...
As far as performing as long as they can keep their vocals/flow strong and body healthy they should go on for as long as they want... I think Elton John has just finished his last tour as his body has finally had enough???
Eventually if you dont show growth you will naturally fade out of fashion anyways and maybe comeback for anniversary shows etc
If youre lucky enough to have timeless music like Rakim, Masta Ace, G Rap, BDK, Mos Def, WU, The Roots, KRS, Tech, JMT and R.A and various other 90s artists etc you can tour Europe and Asia even if America wants to only hear the latest new trends...