Kitten food does contain more calories than food made for adult cats. Kittens are growing and use a lot of energy when they play.
Food for junior cats contains slightly less calories than the kitten food. Junior cats are still growing and sometimes uses less energy playing.
Food for adult cats contains less calories again than the food made for junior cats/kittens. Adult cats need less energy as they play less and are fully grown(this is the type closest to the all stages food, the difference in energy content is only minor).
The food for senior cats contains only the calories needed to maintain the cats weight and often contains a reduced amount of minerals and more dietary fibre.
So you will want to avoid an older cat eating your kittens food or your adult cat may gain excess weight. If your kitten eats the adult cats food it might get less calories but this is not a real problem as the kitten will only eat more often.
The manufacturers of cat food make the different types of food based on an average need for calories in cats at different ages. Most of the cats we have in our homes are not very average. Many cats are good at self regulating their food intake if they get free feeding from a young age but some cats do eat for as long as there is food in their bowl and they need a strict feeding regime where you have to limit the food intake.