The cat food companies start listing 7 years old as suitable for their "senior" products.
Petfinder starts listing any cat over 5 years old as senior (from what I have seen), but I think that is more for organizing the cats into categories for sake of facilitating adoptions. Like it or not, most people don't want to adopt a cat over 3 years old. But I don't think a regular person is going to call a 5 year old cat a "senior"...that's going overboard.
Nano is 6-8 years old but I am going to start calling her "senior" the day I eventually see her trade some of her curiosity for the sake of staying comfortable. Me: "Want to go explore?" Her: "No, I think I'll sit down in this warm spot and take a nap."