Basically, delaying puberty so that mental and physical maturity are acheived simultaneously.
Teen sex used to be a necessary thing, back when teen years were fully adult years, when, in another fifteen years, you’d be a dying old man.
Now it’s unwanted because we have to spend so much more time growing up. Our advanced society has charged us with the responsibility of teaching our children much, much more in order to be effective members of society. They have to know about complex social interactions, financial affairs, and, of course, technical career training. This all takes at least twenty years. In the meantime, our bodies follow their own pace, and have us ready for reproduction five to ten years before mental maturity is reached.
If I understand the OP correctly, the idea of birth control in the water is not meant to be universal birth control, to control who can and cannot have children, but as a selective birth control, that would preven people who were not yet recognized mature adults from having children.
That is, no more teen pregnancy. Only from, say, the age of eighteen up could people have children.
With that limitation on it, I would agree.
I would modify it, however, to a slightly more practical method: at birth, all children (so long as they are born in a hospital, of course), receive an implant that will not activate for ten years, but will, at that point, begin producing a contraceptive. Of course, I would only agree to it so long as the contraceptive was proven to have NO effects other than contraceptive. The child grows up healthy, happy, and without making children of its own until it’s an adult.
As to logistics, if 90% of children are given this contraceptive device, then most teen pregnancies would be prevented, yes? I would understand that perhaps those most likely to have children as teens would be those most likely to not receive the treatment. That, I feel, would be the case with any method.
I sold my soul to Satan for a dollar. I got it in the mail.