Age minus 12 years, perhaps? To my mind, someone who spent the first 16 years of their life in Ruritania, say, will remain a Ruritanian forever, though obviousl

Age minus 12 years, perhaps? To my mind, someone who spent the first 16 years of their life in Ruritania, say, will remain a Ruritanian forever, though obviously there are contexts in which "Elbonian" would be taken to mean a "person travelling with an Elbonian passport" rather than someone who was brought up in Elbonia.