Playing 'brain-training' games, doing crossword puzzles or learning a language might cover up the brain degeneration caused by dementia but can't stop it when it gets past a certain stage. Researchers from Rush University Medical Center in Chicago studied almost 1,200 older people over a 12-year period. They developed a scale to measure people's cognitive activity and tracked them to see whether they developed Alzheimer's. The researchers found that increased cognitive activity was associated with a 52% drop in mental decline over the first six years of the study. However, once people developed dementia their rate of decline accelerated by 42% for each point on the cognitive activity scale. The researchers thought that the cognitive activity allowed people to compensate for their brain's degeneration early on. However, by the time they developed symptoms their brain was in a much worse state and so they appeared to decline more quickly.
You can find out more about this research by clicking on the link in the title of this post.