Your chances of reaching 100 are probably greater than you imagine. It may sound fanciful, but a 100-year lifespan is well within the bounds of probability. That could have profound implications for your retirement planning. While the average 70-year-old man now has a 4.1% chance of reaching his 100th birthday, an average 40-year-old woman has a nearly 13.8% chance of becoming 100. And the number of people who are likely to live well into their 90s has soared.