Well time heals and full healing may be possible it really depends on the person and what they do after they leave the cult.
A study found that christian fundamentalism shrinks the brain but the brain has neuroplasticity which mean with proper stimulation and proper diet one begins making more brain cells even if older, but the younger you are the more new brain cells you make as this declines with age. So I would conclude that the younger and sooner you leave the better your chances of a full recovery and even beyond that. Big changes in your life can a have a very good effect on the brain as it forces out of ruts we have developed through boring repetition and repeated thought patterns making the brain more functional by making weaker seldom used neurons fire which makes them more healthy opening up brain processing power.
http://www.wnd.com/2012/03/study-links-being-born-again-to-brain-atrophy/
http://www.telegraph.co.uk/culture/books/10914137/What-God-does-to-your-brain.html
https://en.wikipedia.org/wiki/Neuroplasticity
Neuroplasticity, also known as brain plasticity, is an umbrella term that describes lasting change to the brain throughout an individual's life course. The term gained prominence in the latter half of the 20th century, when new research[1] showed many aspects of the brain remain changeable (or "plastic") even into adulthood.[2] This notion contrasts with the previous scientific consensus that the brain develops during a critical period in early childhood, then remains relatively unchangeable (or "static") afterward.[3]
Neuroplastic change can occur at small scales, such as physical changes to individual neurons, or at whole-brain scales, such as cortical remapping in response to injury; however cortical remapping only occurs during a certain time period meaning that if a child were injured and it resulted in brain damage then cortical remapping would most likely occur, however if an adult was injured and it resulted in brain damage, then cortical remapping would not occur since the brain has made the majority of its connections.[4] Behavior, environmental stimuli, thought, and emotions may also cause neuroplastic change through activity-dependent plasticity, which has significant implications for healthy development, learning, memory, and recovery from brain damage.[4][5][6]