I don't think , at least in my case, that it's been an old age thing. I had plenty of days back in my working life that I felt it tough to "put one foot in front of the other". I think it's a natural part of life. Some folks blame it on bio -rhythms?
If it doesn't become a daily event, I probably wouldn't worry. If it's persistent and only started recently then maybe a call to the doc would be in order.
It could be anything from poor hydration, nutrition and/or fitness as much as a physical disease or condition. I know someone in their mid 80s that were prescribed physical therapy to be strengthened up after other tests came back negative. They told the patient they were weak. The therapist told them the only limitations on senior fitness are pre existing conditions like injury, disease etc but a senior can do fitness training.