As an old football player, is a weak knee normal?

Yes. It is extremely common, especially if you were a young football player. There is a lot of pivoting in football, as well as strain on the knees. Minor injuries occur to the meniscus (cartilage) that accumulate over time. Good quad strengthening and hamstring stretches can stabilize the knee.
Football. Weakness implies muscle strength loss. This can happen if you are not exercising. Football and other activities in our youth can lead to injury of the knee (cartilage and ligament mostly). As we age, these "old injuries" can lead to progressive deterioration (wear and tear) and arthritis, causing pain and difficulty with exercise. Try low impact exercise such as swimming biking, or elliptical.