PSMKay,
Well, I started to work professionally when I was still pretty young, but I agree it is a long career (so far; more to come). I don't think dance stage managers specifically have an easier time sustaining their careers. It can actually be quite difficult if one tries to limit one's career to one discipline - I think I had a number of 'right place at the right time' circumstances that lead me to long company contracts. Maybe any stage manager that is able to maintain a long relationship with the same company or producer stands the same chance of longevity. I am working for a large company now, which means that some of the physical requirements are less than if I were still driving a van and loading shows regularly with less support, so that helps with wear and tear.
I have also not really found myself to be the type of stage manager that looks to move further up the administrative ladder. I have done a fair amount of teaching and mentoring, but I still find immense satisfaction in the job I do now.