In estimating the numbers, the first dilemma is trying to forecast how long you are likely to live. Most people look at the age when their parents died and then adding a few years to account for improvements in the technology of modern medicine. While this has the benefit of simplicity, it might be better instead to use a data-base that includes more than just your direct family members.
Financial planners generally use the statistics provided by the Social Security Administration. Certainly for tax planning purposes, the longevity estimates of the Social Security Administration are the most reliable – and the least controversial if ever you must suffer through an audit of your personal income tax returns. Those estimates would give me a remaining life span of 20 years (to age 85) based simply on my current age. (www.ssa.gov/oact/STATS/table4c6.html) However for anyone who keeps their weight under control, avoids smoking and opioids, and exercises regularly, the remaining lifespan will be far longer.
Even the private sector insurance companies expect longer lifespans, based only on current age. Northwestern Mutual Life Insurance Company, for example, forecasts a full 35 years to make me a centenarian. (https://media.nmfn.com/tnetwork/lifespan/#0)
Another approach is to look at the characteristics of individuals who live exceptionally long lives and see what elements they have in common. The Blue Zones study, for example, found that one community in Okinawa, Japan had a very high number of folks living beyond 100 years. The researchers determined that it was the poverty of the region that helped people live longer than their peers – the families were so poor that they had no furniture. As a result, everyone in the house was obliged to move from the floor to a standing position many times a day.
Moving beyond anecdotal insights, Boston University School of Medicine started the New England centenarian Study/Long Life Family Study. They studied 5,000 people belonging to about 550 families, all of whom had many members who lived well into their hundreds, looking at questions such as daily exercise, food and wine consumption, cholesterol and levels of blood pressure, and family history of heart disease. This valuable study, funded by the National Institute on Aging (part of the National Institutes of Health in Bethesda, Maryland then devised an online questionnaire. www.livingto100.com. Living to 100 estimates my lifespan to extend to age 104. You can be sure that I am now planning how to use my extra 16 years!