Let me reiterate:
The human population has increased to proportions that exceed the planet's limit (as in we have too many people already and have begun to drain the planet of many resources and are actually depleting the planet of biodiversity because of our expansion)
Reproduction is necessary for the survival of any species
Reproduction is NOT necessary for the survival of any one singular individual.
So sure pregnant teenagers (or any other human of any age group) are useful. But not by the hundred thousands per year in this day and age.
We should probably have all the nations of the world get together and depending on the size of each nation, 1 female per every 5,000 people (men and women) should be given a time slot for when they can get pregnant.