Old World Underground
One of the major benefits to me of a young wife is the cross-generational effect it has on your children. Your daughters are less likely to end up spinsters and your sons less likely to end up beta chasing old hags. It sets a precedent culturally, possibly on a genetic level as far as we know, and you get to have great-grand kids instead of this "I hope kids get to have a grandparent before they all die of obesity at 65" most families are at risk of. It also creates a natural power dynamic within the family, which might have some volatility, but age-gaps do decrease the gender blurring androgyny established by modern society.