While Grade 2 is not usually operated on with an older pup; my surgeon does recommend it in the young ones. Not sure what the cutoff age is. The reason for that is that a luxating patella will wear on the cruciate ligament over time....so that when they are older, the surgeons don't recommend the surgery because at that point the ligament already has some wear and tear. Of course some pups never do tear that ligament but it can and does frequently happen. If they do surgery on luxating patella, often the pup will tear the ligament and necessitate another surgery...more pain for the pup and more money for the owner. In a younger pup, if the luxating patella is repaired, it reduces the risk of a torn cruciate later on.
I always recommend that a board certified surgeon see a pup because they are less apt to recommend surgery than some regular vets (some of which are too eager to operate on LP).....BUT they also may recommend it in a young one for the reason I stated above.
A link about cruciate tears:
http://www.yorkiehavenrescue.com/cruciateligament.html