Ammaron urges Mormon to take the plates when he's twenty-four years old.
Mormon tells us that he's sixteen when "three hundred and twenty and six years" have passed away. Therefore, when he finally in the "three hundred and forty and fifth year" is able to get to the plates, he's thirty-five, and a decade late.
Why does Mormon give us these details? So we'll appreciate the scale of the war? So we'll see him as frail? Conversely, so we'll see that he's struggling to do his best, despite the bad situation?