Everything in existence has its' life cycle growth, maturity and decline. However, who is the next person to decide when someone should and should not retire is my concern. Had some been able to retire Clive Davis we may have never heard "Fallin" or any of the other hit songs Alicia Keys wrote and sung. -As far as the rapper who is 45 years old still rapping at local events who has never been signed by now one may know that being signed is not the end all be all, especially in todays music business climate. Perhaps the 45 year old rapper has built a local or regional fan base, enjoys what he/she does, sells products and merchandise and is happy doing just that. Nope, they shouldn't retire from that.
Come on now retirement is a direct reflection of age be it 25, 35, 45 or 65. You just set the stage for age in your example by stating that a rapper who is 45 years old...
My response to when should a person retire is when they are ready to. I don't think the next person has a right to ask such a question unless they are asking for personal guidance in that case the question may be asked when should I retire.
If one never achieved a certain level of career success what are they retiring from? In the example of a 45 year old rapper who didn't get signed I liken that to someone who has been turning in job applications, but never got the job how can they retire if they never got the job? However, if the person is independent and doing their thing then they may ask themselves when should they retire?
When do you think someone should retire from the music business, destroy their music industry contracts and keep it moving?