The age kittens are available will depend on the source. As a kitten has a lot of life skills, socialization and general good manners to learn from it mother and litter mates, caring registered breeders will not normally let their kittens go off to new homes before they are at least 12 weeks old. Some breeders will not let even them until they are 16 weeks old.
Other reasons for this is to enable the breeder to complete any health treatments, like vaccinations and also to ensure the kitten is actually ready to leave its Mom and litter mates. An older kitten has a more mature immune system and is less likely to be vulnerable to opportunistic diseases and infections.
Lets face it, we all want a cute and teeny little baby, but in the end a few more weeks don’t matter much over a lifetime. You can be sure that your kitten is getting the very best start possible by leaving her until she is old enough, both socially and health-wise to go out into the world.
If you are buying from a shelter then you will possibly find a kitten younger, as shelter kittens usually need homes urgently.
You might like to consider buying an older cat rather than a baby kitten. Some benefits are that a more mature cat will have already been through the baby-learning stuff, like what not to climb and scratch on! You are assured of their personality as it is well developed, so what you see is what you get.
Often an older cat will not be as energetic as a kitten, so will need less time spent with them, if you don’t have much. If you have very young children and/or a baby or, then a more mature cat is a much safer option for both your own children and the cat!
Adult cats are often available from registered breeders once they have finished their breeding career. Some of these may be quite young and only have had one litter or two. Other times a breeder will “run on a kitten”. This just means keeping it until it has reached adulthood to see if it will come up to their particular requirements. This cat may not completely fit what the breeder is looking for, so a very young adult cat will be available, and probably at much less cost than a kitten.
If you have decided on an older cat from a breeder, find a few breeders you like and let them know exactly what you are looking for. Be prepared to wait a while for just the right cat for you and your family.