Well I haven't noticed any major voice changings for rock stars as they get old.Rolling Stones,AC/DC,Ozzy,Iron Maiden,etc all sound pretty much the same,and Steven Tyler's sounds different than 30 years ago probabli because he wanted it to sound this way
But Robert Plant almost lost his voice. And the singers' voice changed over the years, some of them sing a lot lower than before, although it's not bad for them and the production.