I am only in my mid 30s, but losing 20lbs, switching from heavy weight lifting to more cardio/light weight training, taking a few supplements, long stretching sessions and sauna sessions have me feeling more spry than I did when I was 21. Not sure how long that will keep up, but I don't intend to give into inactivity without a fight :p.
Never use them.
Truthfully I believe it's strength training that actually helps keep them up. Could be wrong.
IMHO. Moderate strength, moderate cardio, Moderate mobility.
Mobility in particular may improve blood irrigation on the joints and help regeneration.
Keep them active with loads that aren't too heavy.