Answer:
It varies depending on the adult's level of education and social and occupational roles.
Step-by-step explanation:
Language can always be developed, even during adulthood, even though it will take more effort than in childhood since a child's brain has more plasticity than an adult's.
An important factor is the adult's level of education since learning contributes to the creation of various neuron pathways and development of skills necessary for language development, such as attention, memory, processing, etc.
Occupational roles are also an important factor since these are the roles which are valued within society, thus making it important or not for the adult to develop the language.