The question “Are the effects of early social deprivation reversible?” is discussed in the Gleitman et al text book (page 543, 1995; page 587, 1999; page 517, 2004)

If, after 6 months of social isolation, rhesus monkeys were paired with normally reared “therapists”, three months younger than the social isolates, then after a few weeks normal play activities between the two emerged.

However, this was not necessarily a complete recovery, as the later of two abstracts of papers giving an account of this work demonstrates.