We have told our oldest who is a young teen, we kind of had to since we all live together and sleep in the same bed. The younger ones just think they have more parents and don't get the concept yet. When they ask or when we think they're old enough we will talk to them.
You know your kids and if they are mature enough to handle the information. As for the child with Asbergers, that a tough one. Maybe consult the health care workers you take him to? Good Luck