Health workers should take advantage of significant life events among the middle-aged to get them to be healthier, new NHS guidance suggests.
People whose children are leaving the nest, those who are going through the menopause, or taking retirement should be targeted to be encouraged to be fitter and to stop harmful behaviours, the National Institute for Health and Care Excellence (Nice) said.
"These are times when people may consider adopting new healthy behaviours," according to a new draft guideline from Nice, which sets out best practice for the health service.
Health professionals should spot those who are going through these life-changing events and encourage them to stop smoking, become more active, drink less, eat healthier and maintain a healthy weight, Nice said.
The draft guidance also states that more must be done to tell the general public that becoming ill is not an inevitable part of ageing.
The Government, Public Health England and NHS England should run campaigns to highlight factors that could reduce a person's risk of developing dementia or becoming ill in later life.
"Ill health in old age is not inevitable," the draft guideline states.
"The risk of developing dementia, disability and frailty can be reduced and, for some, it can be prevented altogether or delayed - and the severity of the conditions reduced.
"Smoking, lack of physical activity, alcohol consumption, poor diet, and being overweight or obese are risk factors for dementia, disability and frailty.
"The earlier in life that healthy changes are made, the greater the likelihood of reducing the risk of dementia, disability and frailty. But it's not too late to start making changes if you are already in mid-life."
Professor Mike Kelly, director of the Centre for Public Health at Nice, said: "This new draft guideline aims to help people to prevent or delay dementia, disability and frailty in later life by promoting mid-life changes.
"This includes encouraging people to stop smoking, become more physically active, drinking less alcohol, eating healthily, and maintaining a healthy weight.
"Everyone now understands that smoking, drinking too much alcohol, being inactive or overweight can seriously damage your health, but what many people don't realise is that these factors also increase the likelihood of them developing dementia.
"But what is really clear from the evidence we looked at is that people who become more active, even when they are a bit older, are far less likely to develop dementia; they are also less likely to develop cardiovascular disease and cancer.
"Obviously the earlier in life that healthy changes are made the better, but it's never too late to start."
The piece of work, which has been put out to consultation, also sees Nice comment on the plain packaging for cigarettes debate.
It calls on the Government to "remove remaining opportunities for tobacco promotion, such as packaging, and film and other media portrayals of smoking".
The Department of Health recently published a consultation on the introduction of standardised packs for tobacco products which will conclude in August.
Tim Parry, head of external affairs at the charity Alzheimer's Research UK, said: "Research suggests that lifestyle choices in mid-life could have important knock-on effects for health in later life, including brain health.
"With research suggesting that a third of cases of Alzheimer's could be prevented, we need to see concerted efforts to develop better ways to encourage healthy living across the population.
"The period of mid-life can often bring many changes for people and can be a time for people to reflect on their own health. There have been mixed messages about what may or may not affect dementia risk later in life and it can be hard to know what lifestyle choices may hold the greatest benefits.
"It is promising to see Nice developing guidelines to give people the best information about how to stay healthy."
Dr Alison Cook, director of external affairs at the Alzheimer's Society charity, added: " When you get to your 50s and 60s there are some big life changes happening such as children leaving home or retirement which can be good prompts to have a rethink about changing the way you live.
"Whether you decide to change your diet or just walk for 20 minutes a day, everyone can improve their chances of avoiding dementia. Making good choices later in life is critical."