My first family had an orphan show up right after they were married and had no kids yet. My daughter had an orphan show up after her peeps youngest natural born was about 16 or so. Another one of my daughters had a game where the peeps had 3 natural born when an orphan showed up, then they went on to have 2 more natural born after that. My other two girls have families with no orphans. All this to say, I think it's a pretty random event.