Just my own opinion but if the dog has a relatively good quality of life, and seems to be happy and content then it is NOT time. Pain can be controlled.

You indicate the pain medication was working but after observing the dog limping you gave it pain medication. It's unclear whether you had stopped giving the dog its pain medication, or if the pain has broken through the medication, and it's no longer effective? There are at least 3 different NSAID veterinary medications on the market. You may wish to consult with your Vet to see if switching to another medication might help. If all else fails low dose Torbutrol (a doggy narcotic) used periodically will relieve pain. Some Vets are hesitant to prescribe it due to the narcotic issue around it. But it's an extremely effective and somewhat sedative drug. Prolonged continuously daily use however can eventually build up tolerance. Discuss this possibility with your Vet.

Outside of the NSAID pain killers, you can try a few other things. Go to Walmart (or your favorite store), and get some Glucosamine+ chondroitin sulphate. If the joint problems are due to the lack of lubrication in the joint, then Glucosamine may help. If it is to be of benefit it must be given every day, and you should not expect to see improvements for up to 3 months. Probably a dosage of 500 mg 2X should be given- consult with your Vet. Side effects are minimal to non-existent. So it's a safe supplement to give. Also start supplementing the dog's diet with fish oil. For example a 70 lb dog should receive 4-1000 mg gel caps 2X daily. Adjust the dosage per the weight of your dog. Only side effect would be loose stools if too much is given. The Omega-3 fatty acids in fish oil have weak anti-inflammatory properties. Which reminds me, when you talk to the Vet ask if a short term 1-2 weeks max treatment with Prednisone to relieve inflammation might be worth a try?

The final thing you can do to help your puppy dog is to make sure that when it sleeps, it does not do so on hard surfaces. Typically a dog with arthritis will lay on the hardwood flooring. Why? Because the temperature of the floor is cooler than carpets, and the cold helps relieve some of the pain. Unfortunately the hard surface with make things worse. If you don't have a "wag bag" or doggy bed already, then get one. Most are sold with syrofoam beads as inside filler. Find one that instead has polyester filler such as those found at the Smith and Foster web site. If you can keep your dog from sleeping on the hard floors that should help.

Maybe not the final thing... since your dog seems to want to engage in play, try doing activities that do not require much physical movement. For example playing on the floor with the dog with its favorite dog toy. If it doesn't have a Kong toy get one and put a little peanut butter inside it. Dogs will spend quite a bit of time trying to get at the peanut butter inside the rubber Kong toy. There's a lot of activities your dog can still enjoy. The most important activity is the enjoyment it gets being around its family.