The authors of 'Occupational complexity and lifetime cognitive abilities' examined the associations between the complexitiy of people's main workplace role in life and their cognitive abilities at age 70, based on a research group of 1,066 people born in 1936. They conclude that 'more stimulating environments preserve cognitive ability in later life' - although they add that 'the continued effects into old age are still debated'. 

Negotiating and instructing

Lawyers and judges fall into the occupations which are classed as offering complexity - the kind of work that involves analysing, comparing and synthesising data or negotiating, instructing or supervising people. These people performed better at age 70 than those who had worked with things. The results held up, even when account was made for the cognitive performance of the test subjects in IQ-type tests at age 11. Source: Neurology.org