I am very skeptical about those future life expectancy predictions put out by the UK Office of Work and Pensions.Age.
Yes, 100 is arbitrary but not at all unrealistic. Couldn't find USA data, but if you're 55 in England today, you have a 12% chance of reaching 100. UK's life expectancy is only 2 yrs longer than USA at birth & if you avoid death before age 1, gun fights, & traffic accidents, & have health insurance, I'd expect USA expectancy for 55 yr olds till death are similar if not longer.
Then that being said, planning to run out of money at 85 or 90 or even 95 seems pretty risky vs. the likely change in lifestyle required to use age 100 - or 105. JMO.
In fact, their calculations assume that a new born has the same chance of reaching age 100 that a 96.5 year old does today. And that an 82 year old has a greater chance of reaching 100 than an 83 year old.
See:
Surprising Probability Estimate of the Day
Also, according to the last US census, 62.5% of centenarians were 100 or 101 years old and only 0.6% of centenarians were 110 or older.
I am an optimist on life expectancy but not that optimistic. . . me thinks they are using linear extrapolation on what actually should be an asymptotic estimate.