I don't think it's too early to begin using joint supplements for prevention. Only Natural Pet has some good ones, too. I actually use a mail-order supplement company called Springtime, Inc. (
www.springtimeinc.com) They make supplements for horses, dogs, and humans.
I use their Joint-health Chewables for my 3 dogs. I have one dog that has luxating patellas (as bad as can be without surgery!) and he does great on these chewables. He still jumps on the bed, couch, runs and plays, etc. I also turned my sis-in-law onto these supplements for her old poodle mix, who limped quite a bit, and he is doing great!
I like these supplements because they have all-natural ingredients. They have chewables and some powder supplements, as well.
I use their Fresh Factors vitamin chewable (also has chondroitin in it), their Joint-health chewables, their Stress-free Calmplex chewable, and their Bug-off Garlic chewables.
Good Luck in your search. There are alot of good supplements out there.