[A-2-C]

Not sure about quantities of phytoestrogens in herbal treas and how they would affect a teenager, but it seems that the logic behind "no herbal NBE for under 18/21" is that phytoestrogens are different on molecular level from human estrogens and, from what I understand, may interfere with production and binding of our own estrogens. And, since teenagers have not experienced a full spectrum of hormonal changes yet, phytoestrogens can do more harm than good.

While on the other hand, BO is not supplying much hormones in the pill itself, but rather makes your body activate its own mechanisms of hormone production if something is hindering it (malnutrition, stress or whatever). I wish I had BO when I was developing.