Cards on the table.

With kittens born recently at a shelter, in foster, or who are very young, we can tell their age within a few weeks, as they develop so rapidly. Sometimes we get adult cats whose previous owners know their age and that's great...but it's not the norm. 

99% of the time, we have zero information about a cat, and their age can only be a "Best Guess" based on their condition, overall appearance, and the state of their teeth. We completely understand that you want this information, and since you will need a date of birth for insurance, so we will do our best. But it is generally only going to be an educated guess from our vets.