My rat drags her back legs?

My rat is 2 years and 8 months old, so she's quite old, however she's still quite perky and eats and drinks well. I have recently noticed that she drags her legs, she doesn't do it all the time and she can still move and bend her legs. As she is getting older, she is sleeping a lot more. I just wondered if the weakness in her back legs could be caused by her age ?? Any answers would be useful. Thank you.