I suspect that most of the issues we mid-lifers experience are related to being generally inactive for years, then doing too much/too soon, rather than a purely age thing. I'm not saying that we will be able to do in our 40's what we could do when we were 20, but that we need to be determined but patient in building a base of fitness before pushing ourselves too far.
I have a lot of friends that have connective tissue injuries (knees and shoulders, mostly). It's almost always due to either a) overtraining, or b) being sedentary for weeks at a time, then taking on some strenuous activity in a short burst.