Older cat getting thin but eating normally?

Don't worry it really is just old age. Think of it like when people get old, some older people will stay at their normal weight and others get thinner and bonier. It's completely normal, I have a cat who used to be quite tubby and as he got older he lost some weight and now he is pretty thin and bony but he eats a lot. The fact your cat is happy and playing proves to you she is fine. I have another elderly cat, 13 years, and he's exactly same as the other one, and I've also had a cat who died a couple years ago who was the same, so honestly, it's totally normal. Your feeding her right, she's happy and active but she's just a skinny cat in her old age, nothing unusual and nothing wrong with it :) xx