How old was the person to ever be a grandfather?

Answer 1

I'm assuming the missing word is "youngest." Or perhaps it is "First". If it is First, I cannot answer the question. So I am going with "youngest"

Because of the immense number of people who have lived and died, we can assume that in at least once in human history, the following conditions have met:

A) At least one man has successfully impregnated a woman at the earliest age a male is considered sexually mature - that is, after the onset of puberty.

B) The woman survives the pregnancy, and the child produced from this was a male, who himself survived long enough to reach sexual maturity.

C) The son of the man in condition "A" impregnates a woman, who carries the child to term.

The onset of puberty for males is determined by both genetic and environmental factors, including nutrition; but despite this variables, can generally be considered to be between the ages of 9-14. Incorporating that the incubation period of both children will be approximately 9 months, we can express the age of the youngest possible grandfather as

(9*12)+9+(9*12)+9 <-> (14*12)+9+(14*12)+9

(in months)

So the possible range for "youngest grandfather ever' would be between 19.5 years and 29.5 years.