According to this BBC News story nobody never learns nothing after they leave school. But judging by the quality of the BBC report…

it is already known that dementia is less likely in people who been (sic) educated for longer.
Previous research has shown that education is beneficial because it increases the number of neural connections in the brain.

…the original Cambridge report probably does not say any such thing. On the other hand it may, possibly. As Dr David Llewellyn himself says:

exposure to secondhand smoke may be associated with increased odds of cognitive impairment.

I suppose it may; or it may not. Tricky stuff this scientific research. Could we at least conclude with some certainty that someone’s research funding is due for renewal?