That's a good question!
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.