Eddie Vedder:
http://www.youtube.com/watch?v=VbhsYC4gKy4 (early)
http://www.youtube.com/watch?v=r5UEkumxNCs (later)
Doesn't sound the same, he has to reach for the higher notes and the songs are all down tuned. Don't get me wrong, i'm a huge fan of eddie vedder, but age has changed his voice.
In point of what you just said, your voice does indeed change when you sing in your 20s vs. your 30s vs. your 40s, espcially when you sing for a living, healthy or not, it wears down. And everyone's voice is different, in vedder he lost some of the control of his vibrato as well as his upper register and rasp control, scott's voice dropped about a 5th in his register and he is fully a baritone and no longer a baritenor. David Draiman had to have vocal reconstructive surgery, Rod Stewart did as well, James Hetfields voice is nowhere near the same. Not saying these are all the same situation, but all voices change over time for various reasons.
Not diminishing healthy vs. unhealthy technique expediting the change, but eddie vedder doesn't sing healthily either, nor does james hetfield he also drinks and smokes heavily same as stapp. Granted scott's technique is FAR worse than eddie's, but 10 years does a lot to a voice.
James hetfield sound nothing like he used to and he's somewhere in his forties as well.
Age is just as much a factor in the quality of your voice as poor singing technique and unhealthy living habits, period.