It would appear that there is no medical minimum.
In general, the parents should be deciding such an important health matter, although there is no clear age for any particular thing (driving, sex, drinking, voting all different and varying by state).
However, I think that birth control is so important to society that birth control implants should be mandatory for females under 21.

I come from a very straight-laced, very conservative family, yet when I was 15-years-old, my mother approached me and told me if I ever decided I wanted to start having sex, to please (almost begging) let her know and she would put me on birth control. Her reasoning was that kids are going to do it when they are ready whether the parents know about it or not and it's better to be openminded and take steps to prevent pregnancy than it is to remain delusional that your kids will never engage in that behavior and end up with a pregnant teen. Of course, she did this without my father knowing. In any case, it ended up she didn't have to worry. I didn't end up starting to have sex until much later.

These days, kids are getting pregnant at younger and younger ages. My advice is to sit down and have a heart-to-heart with your daughter and let her know the dangers of early sexual intercourse, but that if she feels she absolutely has to engage in that behavior, that you will be disappointed in her, but will help her take appropriate measures to prevent pregnancy.

Don't forget that birth control in pill form has other uses. Most importantly, aside from preventing pregnancy, it can regulate menstrual cycles. Many brands are also effective in clearing up acne by normalizing hormones. It honestly can offer a girl/woman a lot of relief from debilitating cramps and embarrassing breakouts, therefore enabling better attendance at school, etc. I think if a teenager or even a pre-teen has begun her cycle and she and her parents are mature enough to have a discussion, the girl should be able to make her own decision! I was on it for many years before I needed it for contraception. But I also recommend researching the MHC studies. There is a lot of suggestion that the pill can inhibit a woman's ability to choose a genetically compatible mate. The studies involved women rating their olfactory attraction to white shirts that had been worn by different men. They then compared their MHC profiles to draw conclusions on attraction and the hormonal impact on it. Many scientists are even blaming the increasing divorce rate on the pill! There are tons of resources online, but Women's Health Magazine has an article that explains it simply and in a very understandable way, which I appreciate since most of us are not doctors or scientists...
here's the link: http://www.womenshealthmag.com/sex-and-relationships/genetic-compatibility?
http://blanktshirts.ca/images/AA%20White%20MENS.jpg

22 years old

Too young is before they are menstruating.

Birth control is unnecessary before a girl is sexually active. The pill has both risks and advantages, as does all birth control why risk the risks before it's necessary?. (more info in my article below) I'd say step one, do you feel your daughter is sexually active? if not she doesn't need birth control. If you do, or suspect she may become so soon the best approach is to read up on the risk and advantages and decide which you feel as best with some runner-ups and then treat your daughter as the mature adult she is becoming. Sit down, show her the options and stress the advantages of the ones you chose. I don't know many teen girls that don't like less extreme periods and clear skin. :)