You may think about the reproductive success of a family, which depends on the reproductive success of sons and daughters.
The reproductive success of the sons is theoretically unlimited, provided they are big enough to win many competitions. To get bigger, it's good to have a larger growth period. Thus, latter maturation.
The (direct) reproductive success of daughters is limited by the duration of their fertile years. Thus, the more years of fertility, the bigger the reproductive success. Thus, early maturation.
Somebody asked for references, so study evolutionary theory:
Darwin, The Origin of Species.
Dawkins, The Selfish Gene.
Futuyma, Evolutionary Biology.
Wright, The Moral Animal.
I truly hope that's enough. If it isn't, just don't pretend to understand evolution, and quit trying to understand biology.