All art is but imitation of nature.
Age is no barrier. It's a limitation you put on your mind.
Age is the first limitation on roles that I've ever had to encounter and I hit that awhile ago.
In fact when you combine stem cell technology with the technology known as tissue engineering you can actually grow up entire organs so as you suggest that sometime in the future you get in an auto accident and lose your kidney we'd simply take a few skin cells and grow you up a new kidney. In fact this has already been done.