Does running help or hurt your health at 55?

Yes but... Should always consult a Dr. First to assure that you are not harming yourself in some ways. Start slow and mostly walk at first. Stretching, either active or passive, are a good idea at any age, but especially when you are older.
Potentially helpful. Running can be beneficial at any age. Muscle toning, protection of the knee joints, weight loss and cardiovascular benefits are all possible; however, running in those with pre-existing joint or other orthopedic problems carries the risk of worsening symptoms. At age 55, you need to consult your physician prior to the initiation of a new exercise program. If already running, then keep going!