Quickly tackled:

A women is fertile around, say 14 years old. In natural conditions (the jungle and all), the average age to give birth for the first time must be 15 or 16. If a girl finds herself pregnant at 15, it'd be nice if she could be paired with a tough guy that would fend off the bears and bring the mammoth leg home. And a 20 years old guy is much tougher than a bloke her age.

So, let's take two similar control groups, except that in one, males and females that get together have the same age, the other where there is the above mentioned 5 years difference. And let's put them to survive in the same sort of merciless jungle. The second batch is likely to do much better for the reasons mentioned above. Thus it is the most efficient, in the setting described. And evolution has a wicked tendency to favor a more efficient solution.

Now, if you want to argue that the two same groups put in our modern society would do just as good, fine. But we were made in the jungle. And our hormonal behaviour is still adapted to that.

Which might make you wonder if we shouldn't tinker a bit with it.