It’s the point of hiding from the world, you can not pass an adopted child as your biological child. One key way to address that in those days was the naming. The child does not have your last name so it’s known that the child is not a biological child. I don’t know of any sort of religiously mandated dates or ages to tell the child, but input from a psychological pov, experts suggest that you introduce the concept soon. Like 4-5 year of age, so it’s not something hidden or seen as something that has to be hidden.