Ah, a subject close to my heart but one which I don't really like to bring up for fear of offending legions of loyal fans....
Actual old age notwithstanding, and placing an arbitrary limit of say 65 years of age on the discussion, personally I can't think of a single singer who has actually improved with age, but I can think of many who have gone spectacularly downhill but who refuse to acknowledge the fact and carry on regardless. I've watched a fair few clips on youtube of certain bands who I liked years ago and whose earlier works still form a huge part of my collection, but whose singers are now little more than an embarrassment and just need to give the whole thing up and retire. I find it very sad and disappointing.