I was determined that both my adopted children would know as soon as possible. Unfortunately my three-year-od son had an auditory processing difficulty and had just had a poor hospitalisation experience. He ran screaming from the room shouting, "I don't want to be a doctor!"
My daughter had no such problems, and her precocious 4 year old talks matter of factly about her Mum being adopted. She recently remarked, "Mummy's mother might have grandchildtren by now." I didn't reply with the obvious.