odds are its a age related thing and arthritis or some joint wear and tear
your vet should be able to recommend some things to try to help, but sadly its just a part of getting old
best advice, is start to allow less climbing stairs and if possible LIFT her into your vehicles as to having her climb a plank or??
the less she moves the better she will be at this point
sucks seeing them get old , I feel for both of you's!
I do know many that had luck with this stuff, but its no time machine! WIsh it was!
https://www.cosequin.com/
and even if you start something like this and see a improvement, your still going to be WAY better off STOPPING as much of the steps and climbing as you can, wear and tear still happen, even if something helps masks some of the pain from it!
also, if her weight is a little HIGH< start trimming back, less weight to carry less pain on joints!