Denna started fish oil pretty soon after coming home - due to slightly itchy skin... it took a while, but I do think it helped.
I think we started joint supplements around the 1 yr mark. If you start them earlier, be sure to find one with ZERO calcium included.
Denna did tear her CCL when she was 5 yrs old... might be a genetic problem, but she has finally recovered (one year of conservative management, no surgery)... I think the joint supplements did help. Not sure if it would have helped more if we'd started earlier or not. If weak tendons is truly her genetic disposition (and her uber-straight legs also didn't help), probably not.
Now (at age 6), in addition to fish oil and vitamin E, Denna gets Nupro Silver, Mushroom Matrix, Green Lipped Mussel and "Platinum Performance" daily, along with about 4oz total of "tendon soup". The tendon soup and Platinum supplements are both new additions, from our PT vet (still rehabbing the CCL tear)... the tendon soup is awesome, but not sure if we'll stay with the platinum stuff or not. I tend to rotate through different supplement blends to see what works and what doesn't, and keep her system on its toes.
Also - Denna was a kibble girl until about age 6 months, when we moved her over to a home prepared raw diet. She's still raw fed.