Firstly, your starting age does have a bearing on your physical training. Your body undergoes a variety of changes throughout your life that change its ability to cope with physical demands. If you start at a young age then you can help prepare the body for continuing to practice in later life, but the older you are before you start the more you have to adapt your training from that done by younger people.
For the purposes of this article, the age range is split into young people (0-15), adult (16 – 28) and older people (29+). These age ranges are not meant as an absolute rule, but as a rough guide only. Individual rates of development often vary greatly.
With young people, the fact that their body is still growing means that they are very susceptible to injuries from repetitive movements and great forces. These great forces, when experienced frequently, distort or disrupt growth patterns which can cause severe problems throughout later life.
To eliminate the risk of these injuries young people should concentrate on fitness, general agility and control of movements, and ignore techniques that require impact or great forces to begin with. Alternatively, they can restrict such movements to a padded environment where much of the forces can be absorbed by mats and pads instead of the practitioner’s body.
Experience of different activities is always beneficial for developing a well-rounded individual and so young people interested in parkour should be encouraged to try other things as well.
For older people, the same rules about being careful about impacts apply, not because it affects growth but because of the great force experienced during an impact. The older the body gets the less able it is to repair itself and the longer it takes to recover from all strenuous activity. Older practitioners need to be aware that improvements will take longer to achieve and need to ensure that they allow their bodies longer to recover from activity. With older people, the emphasis is not on preparation to be able to cope with these stresses later in life, but more on learning ways of moving and training that take the differences into account.
It is also important to remember that not only do physical changes take longer the older you get, but also mental changes do as well. We are at our most adaptable when we are young, in all respects.
The existence of rock climbing as a distinct activity in its own right can also be helpful to both young and old people interested in parkour. Climbing helps develop many useful physical skills such as poise and balance, as well as having obvious benefits with development of upper body strength through movements requiring the whole body. When rock climbing outdoors an even more important benefit comes from learning how to deal with dangerous situations and how to act responsibly and developing this maturity. The main reason it is helpful is because it develops all these skills in a way involving little impact and without the same competitive requirements of other sports, making it ideal for both young and old.
The important thing to remember is that as long as you listen to what your own body is telling you and adapt your training to suit your own needs there is no reason that people of all ages cannot train and improve.