Do you think musicians have a prime - when they are most creative and talented?

I think musicians have a prime as far as their popularity goes. As far as their musicianship goes, I think a true musician is constantly learning. If you are dedicated, then I think you can be a better musician in your 50's than in your 20's.

Yes, but its not limited to musicians. Everyone has little surges of that little something.

I know I certainly have my creative surges every once in a while.

My personal creative time is early in the day, so I get up between 5 and 6 AM then go until I stop being creative... usually by 9 AM.

I think it grows in the person as they age, the accumulation of knowledge, the experience.
At some point, the body begins to go out though. Movement is slowed, reaction time, hearing, what I call inner tempo, different things. But, the knowledge may still be there.
To go by the careers of performers is not a good measure, because of the other variables, like whether or not the record company is pushing the adverts or not.