et has a great point. I'm 52, tore an ACL for the first time in my life a month before elk season, and I was a cranky boy all fall. Actually, still am.
I think the best schools of functional weightlifting (think Mark Rippetoe and Eric Cressey), p90x, Enamait's bodyweight and other books, as well as x-fit and later high-intensity blended schools (Gym Jones and Caveman) all have a lot to offer--probably in roughly ascending order for most of us, but with all being worth a great deal. It makes sense to mix it up, but maybe on a more periodic basis than x-fit--in that it takes a while to get and stay skilled at the movements, and more than one specialist thinks he began to drop back after initial gains with x-fit (and that's what started the whole Gym Jones splintering off from CrossFit, which, along with its role in the movie 300, led to a lot of controversy.
Much of that is what I've read or heard, but I do know that the most intensive schools are on a whole different level in terms of mental fitness, and that as you age you have to experiment. Clarence Bass, old weightlifter extraordinaire, has the best (if longwinded) stuff I've seen on aging, experimenting, and changing approaches.
One thing--even if you don't agree with anything else I've said, check out Eric Cressey's stuff on soft tissue warmup work for injury prevention on his website or his excellent book "Maximum Strength". Your muscles improve faster than your joints--and that should give anyone over a certain age pause.