Puzzle-solving doesn't slow down mental decline
64 year-olds studied for 15 years (498 started, 96 completed).
All participants had taken an intelligence test aged 11.
Those with higher childhood scores were more likely to be intellectually engaged.
People who enjoyed intellectually stimulating tasks had better mental functioning at 64.
But rate of mental decline from that point was the same.
Life-long intellectual engagement adds to an individual's cognitive reserve.
I.e. people can suffer more brain damage before clinical thresholds of impairment are reached.