Engaging in a hobby like reading a book, making a patchwork quilt or even playing computer games can delay the onset of dementia, a US study suggests.
Watching TV however does not count - and indeed spending significant periods of time in front of the box may speed up memory loss, researchers found.
Nearly 200 people aged 70 to 89 with mild memory problems were compared with a group who had no impairment.
The findings are to be presented to an American Academy of Neurology meeting.
The researchers from the Mayo Clinic in Minnesota asked the volunteers about their daily activities within the past year and how mentally active they had been between the age of 50 to 65.
Those who had during middle age been busy reading, playing games or engaging in craft hobbies like patchworking or knitting were found to have a 40% reduced risk of memory impairment.
In later life, those same activities reduced the risk by between 30 and 50%.
Those who watched TV for less than 7 hours a day were also 50% less likely to develop memory loss than those who spent longer than that staring at the screen.
"This study is exciting because it demonstrates that ageing does not need to be a passive process," said study author and neuroscientist Dr Yonas Geda.
"By simply engaging in cognitive exercise, you can protect against future memory loss. Of course, the challenge with this type of research is that we are relying on past memories of the participants, therefore we need to confirm these findings with additional research."
Sarah Day, head of public health at the Alzheimer's Society said: "One million people will develop dementia in the next 10 years so there is a desperate need to find ways to prevent dementia.
"Exercising and challenging your brain - by learning new skills, doing puzzles such as crosswords, and even learning a new language - can be fun.
"However, more research, where people are followed up over time, is needed to understand whether these sorts of activities can reduce the risk of dementia."