Not sure I understand your post unless you meant to say "can't do that very well" rather than "can"?
If you meant can't then it could be something or nothing - It could simply be that your knees and ankles are not as strong as they once were so that balancing is just a bit more difficult. If you are just slightly wobbly with your eyes closed and fine with them open and you don't have any other symptoms then probably no need to worry.
f you go crashing to the ground the second you close your eyes or have other symptoms such as vertigo or dizziness or increased falling / clumsiness then probably ask your GP.
All the best
OH