Years ago I was a gymnast and competed nationally. At the end of my career, I had a stress fracture of the talus in my left foot. I notice it bothers me now and then if I'm on my feet too long or try to play too many sets of tennis. Now that I'm older, am I in danger of reinjuring this spot with overuse?
Stress fractures of the talus bone are fairly rare. Rare enough that long-term studies aren't available for the most part. A recent study from the military may help answer some of your questions.

They reviewed the medical records of eight soldiers who had a stress fracture of the talus. Using X-rays and MRI studies, they found mild degenerative changes in the bone a year (up to six years) after the initial injury.

Only half the patients were aware of any symptoms. They reported mild tenderness after excessive activity. No one had any tenderness when examined by an orthopedic surgeon. The long-term results are unknown at this point. Over time, these arthritic changes could progress from mild to moderate to severe but there's no way to know that without further study.