I thought this as well. I think California has some child labor laws which limit the number of hours a child can work per day. So, television shows featuring

I thought this as well. I think California has some child labor laws which limit the number of hours a child can work per day. So, television shows featuring young children as main characters have an incentive to cast identical twins for a single role. Let's say each child is allowed to "work" 6 hours per day -- that's a total of twelve hours/day during which production requiring that character may happen. So, I had presumed when reading the title that adult stars might have gotten their start as children when there was a significant competitive advantage to being a twin. However, given my limited knowledge of Hollywood stars, I don't think this effect plays out in practice.