Somewhere I saw a comment about high kicks and just wanted to add this. At 34 I am still very flexible and can head kick with ease. Now, when I was 20 I could do full chinese splits, not so anymore however proper stretching, proper stance training, and proper practice should allow someone to continue this practice. My Sifu is 62 and can still head kick with ease.
On the other hand, some people are just not built for these kind of kicks. Not saying that by working at them they could not do them eventually, a person's natural build is very indicitive of how well these kind of kicks can be done. The most important kick to learn is the power roundhouse thrown to the legs, which pretty much anyone at any level of flexibility can do. Donkey and stomp kicks as well as reeps and sweeps are very good to learn for those less flexible also.
I also believe that those who spout off about how high kicks don't work and can't be done are either not athletic enough to do them or have been brain washed by the MMA crowd who like to say these techniques don't work, when the reality is most of them have never trained with someone that experienced in kicking arts.
A friend and training partner who trains in and has competed in Muay Thai always used to tell me how TMA like Karate and Taekwondo had weak kicks and their "fancy" high kicks did not work and were uneffective. The first time we sparred and I nailed him with a hook and ax kick that put him down he told me he did not realize those kicks could pack so much power.
In the end, kicks, just like any other technique, will only be as good as the person working hard to perfect them.
"The hero and the coward both feel the same thing, but the hero projects his fear onto his opponent while the coward runs. 'Fear'. It's the same thing, but it's what you do with it that matters". -Cus D'Amato