I've grown up with cats, and not once can I tell how old a cat is by looking at them. In dogs, it's common to tell because their face begins to have whiter fur specks and their eyes are always droopier. But with cats... I can't ever tell. Do they have the same white specks? Do they just sleep a lot more than usual? What makes them older?
So how can you tell the age of the cat by just looking at them? What makes them look older?
I can tell kittens from adult cats, and adult cats from really senior cats, but that's about it. My cat's vet once told me that it can be very difficult to tell a cat's age because they often base it on the wear of their teeth. This, of course, can vary depending on the type of food the cat eats and their other feeding habits. So, when I asked her about my cat's age, they guesstimated 2-4 years old, which isn't exactly precise, but I guess I would know that he's a young adult.
From personal experience, senior cats tend to sleep more and not be hyperactive like kittens. I've also noticed that their fur tends to become stiffer and they walk more gingerly, maybe?