My wife fosters kittens. Often, she'll have a litter of kittens and the mom cat. The kittens often knead, but the mom often will too.

Thanks to evolution, mom cats are hormonally rewarded for taking care of their kittens. When the kittens are nursing on mom, she often looks totally blissed out like a raver in an ecstasy puddle, eyes half-lidded, staring off into space, paws kneading away at nothing.

I assume that kittens evolved to knead because it stimulates milking and then cats just make that association with the act and feeling good, so they retain it later in life.