[jacob171714]

You've got a like 1/9 chance of dying by 21 I've heard. It's probably wrong but a decent amount of people die before they are able to have kids so they add a 0.1 to the replacement level.

[glimshe]

More like 1-2% in the US, you can check it using an actuarial table: https://www.ssa.gov/oact/STATS/table4c6.html