Kids. They grow up so fast these days. But that may not just be colloquially true; it appears that youngsters actually are reaching puberty at an earlier age than they used to, according to reports.
Part of the reason may be sedentary lifestyles. I know my son, for example, spends most of his time sitting around on the couch playing video games or chatting/text-messaging with his friends. My niece is pretty much the same way, spending all of her time on a computer. It turns out physical activity increases melanin in the body, and melanin—among other things—can act to delay puberty.
Other studies have shown that teens in the U.S. hit puberty a year earlier than Danish counterparts, or that wealthy South African girls reach full puberty a year later than their poorer counterparts. From these studies, scientists have concluded that perhaps synthetic chemical factors in industrialized countries and regions are interfering with the endocrine system, causing children's bodies to start the puberty process earlier than they would otherwise.
How much of a problem is this, really? Well, studies have shown that girls who go through puberty earlier have higher incidence of breast cancer, drug abuse, violence, unintended pregnancies, problems in school, and mental health issues. If you don't think those are problems... well, you need to have your head checked.
You can't do much about the chemical factors, unfortunately. But you can get your kids out and get them exercising. Get them in shape. You'll not only be improving their overall health, you may be delaying puberty for them and reducing the amount of time you have to deal with their "teen issues."