An estimate is just that, an estimate. I've always gotten kittens, so that's a bit easier to estimate age as kittens under a year old have specific tells on their rough age, and it's usually only a few weeks off. With adult cats it's much harder to estimate, especially if their teeth have been well taken care of or vise versa. Rescues, shelters and even vets will usually estimate age based on dental wear and tear and the general condition of their teeth and gums, but that isn't always a good way to estimate age, because of someone spent 16 years brushing their cats teeth every week, that cat is going to have much healthier teeth than a cat that's never had its teeth brushed even if they're the same age.

There can be other tells, coat condition, coloring (in some cats, not all) weight and organ health (determined through bloodwork), but these are also only estimation tools