I don't know. I've seen several of the biggies from the 50's over the past 10 years: Chuck Berry, Little Richard, and the Everly Brothers. Can't say they've lost anything vocally. Mentally, yes. Physically, yes. But not vocally.
SIDENOTE: During his performance, Little Richard cleared his throat at one point, saying: "I must have gotten something caught in my throat. That's never happened before." My wife and I were the only ones who got the joke.