In studies of extreme isolationism, researchers tried to determine if there was a limit on age after which an "isolated" child would be unable to develop socially. One child, Anna, was an illegitimate child and became a foster child to a family that had no time to care for her. She was left in the attic often and was attended to just enough to allow her to survive. At age 5 she was sent to another foster home where she experienced more interaction, yet by the time of her death at age 8 her development had not progressed.
In another case, Rebecca, also an illegitimate child, had a new set of caretakers at age 4 and began seeing social development at age 6. Her development from age 6-10 accelerated and she was considered to have normal social development by age 10.
Researchers concluded that social development after extreme cases of isolationism cannot develop past age 5 for a child but can develop before 5.