It is easier to say when a child should not do pointe work, than to give a definite age limit, or specify the length of training to precede it.
Children of the same age group vary enormously in weight and strength. Aptitude and facility cannot be standardized and, as every competent dance teacher knows, these determine the timing for starting pointe work. The most important requisite for pointe work is good coordination of the whole body, each part adapting correctly and without strain to any new position without losing the "placement" which should have previously been established by barre and centre training.
FIRST. Is the child's posture correct? Can she stand and move without losing this posture?
SECOND, strength. Can all exercises on the demi-pointe be performed with the supporting knee absolutely straight? If not, back, hip, abdominal and especially thigh muscles are not yet strong enough. Pointe work at this stage of a dancer's development risks damage to the feet, knee joints and potentially even the spine.
THIRD, feet. (a) Are the feet absolutely free from any tendency to sickle in or out? If not, the ankle joints in particular will be subjected to strain. (b) Is the ankle joint sufficiently flexible to form with the forefoot a right-angle when on the demi-pointe? (c) Are the toes being correctly used inside soft shoes, i.e., without any clutching or curling?
It is an unfortunate that in some schools pointe work is viewed as a goal to achieve as quickly as possible. In serious schools of ballet it is regarded as of no importance and arises from the years of training quite naturally and at the proper time. The teacher is best positioned to determine when that moment arrives.