Historically, we're underestimating technological progress. What seems impossible with today's standards may be the new normal tomorrow.

Having said that, the specific number is less relevant and it's more about putting things into perspective.

If you believe you won't live past 80, 40 is half of your life and you may think it's too late to start something new. However, if you up the number to 130, then 40 seems like just the beginning.