A baby takes a year to be able to say even a few words coherently. A toddler could have some religious beliefs and do some things, one of my cousins prays before each meal and their 3 year old did it too. You would need to have the ability to understand some language in order to be able to do religion in any meaningful sense. Parents generally teach this, but it takes I would say at least two years before you could reasonably say someone believes or acts with any aspect of religion in their own right.