How Do I Tell How Old My Kitten Is?

my kitten is almost 3 months old. i have had her since she was 2 weeks old. i rescued her from a breeder but decided to keep her so my vet said it would be okay with him if i kept her just fine, its just that he didnt want me to get one of the older ones because they are more likely to get sick and die then younger ones. shes very playful when she wants to be which is about once every couple days or so, sometimes every few hours actually..she loves running around the house in circles…i dont know what age exactly how do i tell? also my Vet can see what they look like when they are born but he doesnt really know their ages unless you ask him for them! does anyone have any ideas on this?