My friends horse has the same thing, but it doesn't seem to affect him at all! He is also 13. He is on cortaflex. At night, you can hear him clicking around the field!!!
My sisters pony who is 26 has that aswell but doesn't bother him, In the winter we put him on cod liver oil and make sure he is warm, He does have a touch of Arthitus (sp) But nothing to worry about.
Everyone seems to panic when they hear a click. personally unless it is a problem i.e lameness, discomfort, or behavioural issues I wouldn't worry too much. Some amount of clicking is nomal however what's normal for one horse isn't for another. A bit like humans, some of us click more than others. Excessive clicking everywhere can be an indication of arthritis onset but one click I would say is not too much to worry about!