I would recommend a vet consult as well. The vet might be able to easily diagnose, or might frankly say that it would take some really expensive tests to diagnose (at which point you might decide not to do the tests).
We had something similar.
My cat (who stayed with my parents when I went away to school/moved out) had some problems with her back legs too around that age (14 or so). It started with some stiffness/weakness in the back legs. She was falling over while trying to use her litterbox. It got to the point where she could hardly walk.
The vet was honest, and said she didn't know what it was, and that it would likely take some very expensive tests to diagnose, and then it might not help anyway (suggested it might be neurological). It might be something incurable, or it might be something that the treatment would be prohibitively expensive.
The vet suggested she be put down, but my mom wasn't ready to do that. Through this all Mom set up the cat's bed, food and litterbox all in one room (the kitchen) and kept a close watch over her (stay-at-home-mom). When Mittens could hardly walk and was falling over in her litterbox, Mom would help her in and out and hold her up so she could use the bathroom.
After a couple weeks (no treatment, just love & support), she made an excellent recovery and was able to walk again and no longer needed support to stand in her litterbox. She lived for another year in good geriatric health.