The scripture that say those who die at the age of 100 are still just a mere youth and are accursed ( because they did not accept Jesus as their Savior before their 100th birthday ). So those 100 years came from Isaiah 65:20.
Isaiah 65:20 King James Version
20 There shall be no more thence an infant of days, nor an old man that hath not filled his days: for the child shall die an hundred years old; but the sinner being an hundred years old shall be accursed.
Doesn't say anything about the sinner dying.
The point is that 100 years old is not very old. But we see it is an age that men will be held accountable.