The Changeling's Choice comes to every changeling child of sufficient strength. It is a simple one: are they human, or are they faerie? Since the strength of every changeling varies, so does the age at which the choice is made. Some precocious children make it at age three, weaker ones hold out until their early teens, and it never comes to the weakest at all.
Whether the child chooses human or faerie, they do not remain long in the mortal world. Those who choose faerie are taken to the Summerlands to live with their fae parent until they reach maturity. They never see their human parent again. Those who choose to remain human are killed in tragic "accidents", and the faerie parent is left to mourn with their human partner.