The most extreme case is represented by feral children. A feral child is a human child who has lived isolated from human contact from a very young age, and has no (or little) experience of human care, loving or social behavior, and, crucially, of human language.
Feral children lack the basic social skills which are normally learned in the process of enculturation. For example, they may be unable to learn to use a toilet, have trouble learning to walk upright and display a complete lack of interest in human activity around them.
Oxana Malaya began her life living with dogs, rejected by her mother and father. She somehow survived for six years, living wild, before being taken into care. There are few cases of feral children who’ve been able to fully compensate for the neglect they’ve suffered.