Is purring in kittens an involuntary reaction?

It's something I've noticed recently in my kitten. If I merely put my hand near her in order to stroke her back or her head, she starts purring as hard as if I've been giving a regular cat a long rubdown. Our second, older cat takes a lot more attention before she starts purring. So I'm wondering if the purring is an involuntary reaction in a cat until it reaches a specific age.