We vaccinate kittens with FVRCP twice starting at 8 weeks of age with the second shot given at 12 weeks of age. A final vaccine should be given at 16 weeks. If the kitten is purchased at 12 weeks old, it is the buyers responsibility to take the kitten to their vet and have third and final vaccine given.
The AAFP guidelines state that you can start this vaccine when the kitten is as young as 6 weeks of age but, unless there is a very high index of risk, I would definitely not vaccinate a kitten this young.
We wait until the kitten is at least 16 weeks old to receive his last kitten shot because the antibodies he got from nursing on his mother will have decreased to a low enough level that his own body can respond to the vaccine in order to make his own antibodies.