Isaiah 65:20 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.
During the Millennial reign of Christ and the saints on the earth, living conditions for the mortal inhabitants will be so dramatically improved that infant mortality will be eliminated and someone who dies at the age of 100 will be considered a child.
Here is a link to information on this topic:
I hope you have found this helpful.
God bless,
Mike