Kitten food isn't a marketing tier. Growing cats need considerably more protein, calories, calcium and phosphorus per kilo of body weight than adults, and they need it in a very small stomach. Feeding adult food to a kitten holds back growth in ways that are hard to make up later.
How long they need it
Most kittens stay on kitten food until around twelve months. Large breeds like Maine Coons and Ragdolls keep growing longer, often to eighteen months or beyond, so go by your cat rather than the calendar.
Switch to adult food gradually over a week or two when you get there. There's no benefit to keeping a fully grown cat on kitten food, and the extra calories will show.
Little and often
A kitten's stomach is about the size of a walnut, so the day's food needs splitting across four meals when they're small, dropping to three and then two as they grow. Big single meals get eaten too fast and come back up.
Wet food shouldn't sit out. Put it down, give it half an hour, then lift what's left. Dry can stay available, but measure the day's amount out in the morning rather than topping the bowl up.
Never cow's milk
This is the one people get wrong most often. Most kittens lose the ability to digest lactose after weaning, and a saucer of cow's milk usually means diarrhoea, which is genuinely dangerous in a small kitten. Water is what they need. If you want to give milk as a treat, use a kitten milk made for the purpose.
Try different textures now
Cats form texture preferences as kittens and hold them stubbornly for the rest of their lives. A kitten who only ever eats one kind of pâté becomes an adult who refuses everything else, which is a real problem when you need to change food for medical reasons at eleven.
So offer wet and dry, gravy and jelly and pâté, while they're young and open-minded. It's the cheapest insurance you'll ever buy.
Don't change food in the first week
If you've just brought a kitten home, find out what the breeder or rescue was feeding and stay on it for a fortnight. New house, new people and new food at once is how you get an upset stomach and a kitten hiding under the sofa. Change once they've settled.
Buy small bags
Kittens eat very little, and dry food goes stale a few weeks after opening. A small bag you finish beats a large one that goes off, whatever the price per kilo says. You'll also find out sooner if your kitten won't eat it.
Two other things
Never feed a kitten dog food. Cats need taurine that dogs make for themselves, and a deficiency causes heart and sight problems.
And if you're setting up a litter tray, use a non-clumping cat litter until about four months. Young kittens taste things, and clumping litter can cause a blockage if it's swallowed.