The joints do get worn out with the age tho..

From what I’ve seen it seems more likely that joints take constant low level wear but at young ages the repair mechanisms easily compensate for everyone but extreme athletes, while at older ages it takes much less to overwhelm the repair mechanisms and start doing long term damage.

What's the best way to preserve them

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.

A mobile lifestyle is good, obesity is bad.

Sometimes I imagine a future where you van replace parts of your body. A knee, hip or ankle replacement, and a yearly revision after 70 (like a cara), and we could live until 200.

Probably this thinking comes from "ghost in the shell" mixed with Asimov's books.

Brand new skin please!

It's not like entropy is a relic of the past.