General rule of thumb is about a year old. However, that does not apply to all cats and all situations. The best practice is to monitor your cats body condition. Weight is kinda misleading on its own as you are looking more for body condition once they get to the 4-6 month stage. Kitten food has higher fat levels and protein levels as a general statement to help the kittens growth. If your kitten is gaining too much weight and getting chunky then it is time to switch to cat food and monitor their diet more closely to prevent having a chunky cat. In my experience the one year has been a good point for mine; it may not apply if you have an indoor only cat, a spayed/neutered cat who is mostly sedentary, a particularly small cat or one of the larger breed cats.
This chart shows a typical body shape ideal so you can help monitor your kittens growth and body condition.
When you do decide to switch you want to do a slow change of food so you don't upset your kitten tummy. A simple method is to fill a gallon bag with the kitten food. As space allows, add a cup of the cat food into the bag and mix. Keep doing until there is no visible kitten food left. The other method is to do a 1/4 cat and 3/4 kitten for a day or two, then 1/2 cat and 1/2 kitten, then 1/4 kitten and 3/4 cat, ultimately ending at all cat. I prefer the mixing method though because it is less complicated to figure out the portions and ends up being a very slow adjustment; my cats have never had any problems switching with the slower method I do.
It doesn't really matter. Kitten food is just typically higher in calories than adult food, however, I never fed kitten food to my cats when they were kittens.
There's really no right answer to that I've learned. It all depends on your preferences. Just make sure to monitor the cats calorie intake to make sure they are getting enough. I was feeding kitten food exclusively from 10 weeks old till about 5 months old, but then she started to get bored of her food so I started rotating some adult variety canned food in with the kitten food, and now at 8 months old, she gets the kitten food maybe once every 3 days now. I also feed some dry and use an all-life stages food since there is actually more calories in that brand I use than there is in most kitten dry foods. I also monitor her calorie intake and currently try to reach 250 calories a day since she is very active.
The vet speculated that my gal was approximately 7 months old when I adopted her, and at that time the majority of her meals was Purina One formula for kittens. The high calorie statement on the label fooled me, more or less. It wasn't long before I fed her adult formulas from Evolve and Purina Beyond. I don't see any ill effects from the adult food.