In neurodevelopmental disorders, brain development is thought to be divided into a series of key stages, with problems occurring early in life being considered irreversible in maturity. However, findings in mouse models of neurodevelopmental disorders such as fragile X syndrome, Rett syndrome, down syndrome, and neurofibromatosis type I suggest that genetic or pharmacological manipulations may be able to reverse certain molecular, electrophysiological, and behavioural deficits associated with these disorders in adults. Studies have suggested that environmental changes or medication can reactivate crucial period-like plasticity in the adult brain. These findings raise the intriguing potential that targeted pharmacological treatments combined with training or rehabilitation programmes may be able to ease or reverse the symptoms of neurodevelopmental problems even after important developmental periods have passed. Despite the difficulty of translating findings from animal experiments to human practise, these findings indicate a rational basis for adult neurodevelopmental disorders treatment.