Singers voices definitely mature with age,i was lucky enough to catch the "Empress of Soul" (twice) on her farewell ( i hope not) UK tour in 2009. She may be 66 years young, but still the BEST Female vocalist on the Planet!
There is a fine line best illustrated by the late great Frank Sinatra. In his 1940s recordings he is pitch perfect with his young expressive vocal prowess and range of sounds. In the 1950s recordings, his voice shows greater maturity with vocal phrasing and spot-on delivery of any song he sang. In the 1960s recordings his voice is that of an older singer whose maturity is clearly evident, but who has clearly learned his trade with a great deal of effort. From a point in the late 1970s onwards, it is clear that his vocal performances are more demanding and harder to achieve and which show a decline in his musical abilities. At that point, he should've considered retiring, but didn't and thus every later recording sounded worse than the previous. So I guess the answer is yes, singers voices do mature with age, but if left then decline rapidly into being unlistenable.
As suggested it is not just the singers voice that matures but us and our choice of music. I listen to stuff now that I would never have bothered with when I was younger. As for my choice of music in my younger days most of it I consider rubbish now.
I used to like listening to Guy Mitchell back in the fifties. So a few years ago (10 or more) when I learned he was doing a tour of the UK and would be appearing in Southport, my wife and I decided we would book up and go to see him in person. What a let down! He was diabolical. Of course what I didn't know at the time was that he was a recovering alcoholic (is there such a thing?) I don't know what happened to him after that but vowed never to go and see ageing artistes ever again. I preferred to remember them as they were. I still sing Guy Mitchell's well known songs to myself and can remember every word of most of them. I've still got most of his records (78's) too
Yes, why not I like the verse that goes "I went to her ma and pa, and said I loved her only, they both said, we could be wed, oh what a ceremony..." etc. ( I think it's the bit about the six baboons bringing out bassoons that gets me....