For the ankle, it is crucial that young athletes limit the amount of side-to-side movement that occurs in that joint. Either during one leg standing postures or when running, the more their ankles roll the better the chance it will push their knees either in or out during faster-paced athletic events. Kids who tend to roll their ankles a lot may be much more susceptible to knee injuries when they get bigger, faster and stronger in their later years.
The hip joint needs to both be flexible and strong to function correctly, making it a little harder to train. For the flexibility side, stretches that specifically target the hips may be needed for those with limited ability to do a deep squat. Very young athletes (ages 11 and younger) are almost never in need of these, but once the teenage years approach and growth spurts really kick in, more stretching may be warranted.
Strengthening the hips can be tricky, because most athletes with weak hip muscles have learned to move in a way that shifts the stress to their stronger leg and back muscles. You'd think a basic exercise like a squat would work the hips very well, but not for those who are leg-muscle dominant already. Isolated strength for the hip muscles plus relearning other exercise patterns, such as squatting, must both be done to stabilize and protect the knees.
Just as important in this equation is for young athletes to learn how to move properly. Being able to efficiently absorb the force of gravity when landing on a jump can lower your ACL tear risk substantially, and is relatively easy to learn for most focused and dedicated athletes. In addition, controlling momentum during stopping and cutting movements will further decrease your risk. These skills tend to take much more repetition to improve on, but it certainly can be done.
Although it is true that the younger someone starts improving these skills the better chance it will lower their future injury risk, it is never too late to build the strength, flexibility and movement skill required to keep your knees stable and safe.
SOURCES: British Association of Sports Medicine,