I don't think there was really an age for us. Our oldest is 4 and he understands how we flew to Guatemala and picked up his baby brother and sister. He understands he was born in Guatemala and we flew there to get him just like his silbings. We talk about Guatemala all the time and how we became a family. We just incorporate it when it is appropriate, but we don't "dwell" on it, because we don't want him to feel "different" either.
Just my opinion.
