How great that your so active at 73. The Naproxen can take a couple of weeks to build up in your system, so I would give it another week, and if your knee isn't improving by then I would go in. Its probably something simple-if it were serious you would be getting more than a twinge-but you don't sound like someone who is willing to sit around or rest too long. If you aren't icing regularly, I would recommend that 4 times a day for about 20 minutes each time. If it is just bursitis or something else soft tissue you will find it takes away the pain for a while and will help with swelling, even if it is minor.
You don't say if you noticed it after overdoing things or an injury, so it sounds as if it just slipped up on you. Hopefully it will go just as quickly. If not, don't ignore it. Find out what is going on and get back to your active life.