I own Yamaha baby grand piano. And today, I forgot that I left it open after practising until I caught my cat treading on the strings. My cat isn't that heavy (he weighs 5kgs), but he plodded on the strings quite heavily that sound came out of the piano. He walked on the strings and the dampers. I am not sure if it can cause any damage but if it does, what kind of damage will I expect? I am also not all that sure if I should call piano tuner because the piano doesn't sound out of tune. But just for precautions, will the piano strings weaken and eventually snap after the fact?
The hammers hitting the strings, particularly on sfortzando playing, will stress the strings far more than a cat gently pussyfooting around on the strings.
A bigger problem will occur when he finds the felted parts on the dampers are great to get his claws in, rather like a scratching post. He'll love it, you won't.
If it has to stay open, then I suggest you hold down the damper pedal, as walking on the strings then will cause vibration that may make the cat get off. But that's a desperate measure. Close lid, close door to room, or incarcerate cat !!
And - having a piano for seven years doesn't make it relatively old. A relatively old piano may be a hundred years old.
I agree with Tim however I will remind you that cats have extremely sharp claws and 5 kg might not damage the strings but maybe the tuning by really minute amounts every time. Also I suggest you just wipe the strings with a dry cloth. Dampers are really susceptible to cat's claws and therefore you better keep your cat away from it.
P.S. The piano is too heavy and large to be kept away from your cat.
Also check the model number. I am not sure where you can find it but I think maybe on the Lid of the piano. Old pianos are worth a lot of money in which case even frequent untuning by your cat can spoil it. I am sure many people here could tell you where to search for it or just ask your tuner to help you the next time you see him.