On days when my mother didn't seem to have her usual level of energy, she's say that her "get up and go had got up and went." Did you ever have a day like that? You aren't lazy. Most of the time you can do tons of things - all day long, but not today. Ugh! For some people those days become more frequent and they just shrug and say it must be due to old age. But wait! It very likely is something else.
Yes, it is common for us to slow down with age but not all of us do. There are many elderly people that we can describe as active. When was the last time you took stock of your overall health? Perhaps there is a physical or mental cause of your fatigue. Did you just eat a big meal? Overeating can certainly weight you down and give you that feeling of listlessness. Obese people can often experience fatigue - having to carry a lot of weight is tiring. There are numerous other possible physical causes. Are you getting enough sleep, of instance? You might also have psychiatric issues such as job worries, drug or alcohol abuse, simple boredom, relationship problems, or grief.
You cannot be productive, and you certainly cannot change the world if you are unable to function. If you experience a day where your get up and go has got up and went, don't panic. It might just be a one day thing. If it seems to be happening more and more though, do something about it! Look at the possible causes and see if you can eliminate any problem. Seek professional help though if it turns out to be too big to handle alone.