By and large most millennials were children of boomers. Even a boomer born in '46 would only be 19 and average at first birth hovered around 24 for their generation. Most kids born from 1980-1991 which covers 75% of the years millennials were born had boomer parents. Math isn’t on your side and you keep saying Gen Xers did it, but what proof do you have?
I also imagine you prep very differently for an 8 day mission vs a 6 month mission. In terms of physical fitness, trainings, family life etc.