Our vet recommends senior checkups starting at about 8 years old.
Individual appearance of aging & age-related problems seem to vary widely in cats I've had.
My rainbow bridge tortie girl started having health issues at 11, and lived to be 17. She started slowing down & beginning to look frail at 13 or so. Our 16-year-old tuxedo girl is only now beginning to slow down just a little bit, though she's still quite playful and has a gorgeous, plush coat. We started noticing a slight change in her eyes when she was about 10- they get sort of a 'milky' look to them in certain lights. Our other two 10-year-olds act & look almost like they did at 5 - same with our 8-year-old cat.
Good preventive care & regular senior checkups can catch problems early & really make a difference.