Every retirement calculation needs an end date, and it is the one input nobody can supply. The way that uncertainty is handled shapes the plan more than any assumption about returns.
Life expectancy is an average, not a forecast
A published life expectancy is the mean outcome for a large group. By construction, a substantial share of that group lives longer, and some live considerably longer.
Planning to the average therefore builds a plan that fails for roughly half the people who follow it, which is an uncomfortable property for a spending schedule.
The relevant figure for planning is not the average but the tail, meaning the age that only a small minority exceed.
Actuarial tables prepared for pension and insurance use present exactly that distribution, showing the proportion surviving to each age rather than collapsing everything into a single headline figure.
Expectancy rises as you age
The figure quoted at birth includes everyone who dies young. Someone who has already reached sixty-five has passed those risks and has a higher expected age at death than the headline suggests.
This conditional nature is widely misunderstood, and it causes people to underestimate how long a retirement starting in their sixties might last.
The same effect continues throughout retirement, so remaining expectancy declines more slowly than each passing year would imply.
Couples face a longer horizon than either individual
Where two people depend on the same savings, the money must last until the second death rather than the first.
The probability that at least one of two people reaches an advanced age is meaningfully higher than the probability for either alone, which extends the planning horizon.
Plans built around a single lifespan routinely understate this, and the shortfall appears late, when the surviving partner has the fewest options available.
Pooling is the only real solution to the tail
An individual cannot diversify their own lifespan, because they experience exactly one outcome. Insurance works by combining many people whose outcomes differ.
Lifetime income products and state pensions operate on this principle, using the contributions of those who die earlier to fund payments to those who live longer.
That is why guaranteed lifetime income is expensive relative to a fixed-term equivalent, and why it addresses a risk that investment returns cannot.
Health does not translate cleanly into a number
Family history, current conditions and lifestyle all shift the odds, and some income products price these differences explicitly.
They shift probabilities rather than provide certainty, and individual outcomes vary widely around whatever adjustment is applied.
Anyone weighing these factors for a specific plan is dealing with medical and financial judgement together, and the specifics vary by jurisdiction and change over time.