Neurologist and professor at the University of California Irvine Dr Claudia Kawas is searching for the secrets to living a longer and healthier life.
Dr Kawas and her team have been running a major study of ageing California residents for more than two decades. She discussed the study and shared some of the secrets that can lead to having a longer and more active life with Sunday Morning.
She says estimates range from 30 percent to 70 percent for the importance of genes in a person's longevity, however, environment matters at least as much.
Much of her research has taken place at a Florida complex, Laguna Woods, which has about 18,000 elderly residents and a choice of 400 clubs they can join.
She has surveyed them on lifestyle - diet, exercise, activities and medications. Findings so far include that no vitamin supplements make a difference, modest coffee and alcohol intake help with having a long life while her "favourite one" is that gaining a small amount of weight is also a positive.
"Losing weight is not good, being thin is not good when you're in your eighties and nineties."
Gaining about two and a half kilos every decade past 65 is associated with the best outcomes in terms of mortality, Dr Kawas says. Some reserves of fat and the vitamins that stay in there may be an advantage, especially if the person becomes ill. It remains a mystery but she is certain that a little bit of "extra padding" is helpful.
More than 2000 people in their nineties have enrolled in her study, "Ninety Plus". They are interviewed in person every six months about their mental and physical functions, they undergo tests including blood tests and MRI and CAT scans - and they are asked to donate their brains after they die so that further research can help other people age well.
While some people can't remember things or care for themselves very well, there are some at the other end of the spectrum who are "stunning".
As little as 15 minutes a day of exercise can help with living longer, but the maximum benefit gained is from 45 minutes a day, she says.
Those who live the longest are not spartan in their approach to life and are proud to say so.
"Attitude matters. Attitude is also associated with longevity and I think most of the people who have made it into that age have really shown some kind of spark that I think has a lot to do with it and depriving themselves or being too strict in any of those parameters is generally not a feature we see in these folks."
'It's not healthy to not engage'
Socialising and participating in activities that we enjoy are crucial, she says.
"It uses a lot of parts of your brain; it moves it along. I think it makes all kinds of connections and changes all kinds of hormones and I think at the end of the day on average it is a very important part of brain and body health.
"It's not healthy to not engage with people."
Those taking part in the "Ninety Plus" study are not representative of the general US population, she says. The people are mainly white, have a higher education level and have a higher socio-economic status.
Another study happening in Northern California, "Life After Ninety", covers the same age-group and includes more Blacks, Asians and Latinos. However, some aspects of her study such as the value of exercise would apply for other ethnicities.
The study of participants' brains has revealed that 40 percent of people who die without dementia have enough Alzheimers pathology in their head that doctors would have thought they should have had dementia. This is a stunning result, she says, because they had been functioning normally in their daily lives.
On the flip side, some people who had been diagnosed with dementia did not have visible signs in their brains when looked at under microscopes, which meant doctors were missing something.
"There's a lot more going on... I think we need to understand why these pathologies are more toxic to some people or why some people appear to be resilient to them and can keep on going and others can't."
After age 65, the chances of getting dementia or Alzheimers doubles every five years but she has met people as old as 108 who have maintained "good cognition".
Dr Kawas says many of the people enjoy taking part in the research, and want to contribute to the welfare of future generations even if they don't have their own children. She asked one man if he would be willing to donate his brain when he died and he replied: "Sure. I won't need it then."