In olden days these Asanas were practiced monasteries and so the people were strong and healthy and had long life. In school and colleges these Asanas are introduced now days. The postures are as many in numbers as there are numbers of species of living creatures in this Universe (84 lacs of body, there are 84 lacs asanas describe by lord shiva. Among them 84 are best and among these 84.32 have been found very useful for mankind.
There are some which can be practiced while standing, sitting, lying down, inverted etc.Generally after age of ten or twelve these asanas can be practiced. People of twenty or thirty year of age perform all asarana nicely. Practice for month or two will render all rigid tendons, muscles and bones very elasticl.Even old age can practice all asanas, they can give up head stand if they are not physically fit. There are persons of advance age who practice head stand also.
In India there are the monks who fallow the non dualistic philosophy, they afraid to do asanas(postures) and pranayams(breathing practice) on the ground that the practice will intensify goddess and militate against his practice of non attachment. Though the two paths hatha yoga and non dualistic are entirely deferent yet a non dualistic can harmoniously combine postures and breathing to his best advantage. We have seen many non dualistic sickly conditions with very poor physique and dilapidated constitutions. They can very hardly any non dualistic practice. The body is closely related to the mind. A weak sickly body means a weak mind also. a non dualistic do a little practice of asana and pranayama to keep up his body and mind strong and healthy, he can do his best practice of meditation to reach the supper conscious state. If these two product (postures & breathing) breaks down, you will not reach the goal of self enlightenment.