I'm afraid there's no definitive answer, because every cat is an individual. Most times, we'd consider cats aged 7-10 to be middle aged, 11-13 to be "senior" and 14+ to be "geriatric." That's just an average ballpark, though.
After all, some cats still act and feel like kittens at 12 years old, while others act like crotchety old lady kitties at 6 or 7.
p.s., I'm delighted to be here! Took me a while to log on, and figure out the 'puter stuff.
My first cat Ginger is 7 this year and she has gotten so moody, she does not play with the kittens anymore and she will not even play with me! and I used to be able to pick her up whenever i wanted or do anything to her, now if I pet her and she does not want to be petted, she will swat at me!
Any change in behavior could potentially be due to a health issue. 7 isn't really all that old. But depending on what's going on in the house (kittens? how old? rambunctious/pestering/etc?) even a 7-year-old kitty could start acting like a curmudgeon. If the other cats are suddenly challenging Kitty's authority, that may also be reason for her to start acting differently.