I turn 45 this month (and I celebrated by running 45 miles two weeks ago!
http://timothychenallen.googlepages.com/occ45miler.html). My primary sport the last three years has been long-distance running; specifically ultramarathoning. But I recognize that I'm over-specialized; I wanted to branch out. I saw a description of Parkour in a book; googled it and saw the videos and immediately knew that I wanted to try it.
I'm fitting all of this in around being married, having a seven year old son, and having a very time-intensive job. Fortunately, I've become better about time-management with age. Unfortunately, my energy levels sometimes let me down. I
do find that once I get started, I can overcome being tired to have some fun.
Right now I'm doing the
Workout Of The Day and have been working on some of the most basic moves-- rolling and precision jumping. When I'm comfortable with those, I'll start working on vaults. The point is, I'm not in a great hurry to learn everything at once. The workouts already are very different from what I am used to, which is a really good thing. I've really lost a lot of upper body strength, so working on pulllups and pushups (two of the exercises in the APK warmup) is really good for me.
I agree with the discussion so far on the topic of injury: when an old guy gets injured, he stays injured for a lot longer time! When I was 25 I used to do dumb stuff knowing that if I got hurt I'd be up and around again in a week. Now it's more like a year. So I'm a lot better at not hurting myself now. The trick is to balance that with still wanting to try new things and take risks. I mean, there's not much point avoiding injuries at the expense of always playing it safe!