I'm 57, and I don't have the kinds of aches and pains some of you 40 year olds are talking about. There are a few things that are key for me.
First, I keep my legs in shape. I run most of the year (not at the height of summer though), 3.5-4 miles three or four times a week. I play golf, walking the course carrying my bag. When it's hot, I ride my bike for 30-45 minutes at a brisk pace (20-22 mph).
I also do pushups every day. 20 reps 3 time a day. It doesn't sound like much, but it really keeps the shoulders feeling good.
Sometimes I hit the gym, but not regularly.
Other than that, I play tennis and practice hard. Sometimes I am a little stiff in the morning, but that goes away completely after I've been moving for 5 mins or so.