Ballet is a very intensive training and starting young is key. The Royal Ballet School in London takes dancers as young as 11 years old for full time training, and people who undertake the full training course there will be training for 8 full years with the aim of perfecting their dance movement and style so that they can blend seamlessly into the Royal Ballet companies and tour as professional dancers.
The aim is to create a very uniform style of dance and the training scheme is very strict and rigorous. For many ballet schools there are certain height requirements and if you join at a later age you must be within a certain height or weight specification to be accepted. The audition process for these schools is very intense, and the first audition is roughly 1 hour 30 minutes with solo and group performances. Then, if you are successful here you will be invited to a secondary audition.
Often these schools offer boarding school positions for their applicants so that they can be together all the time and really get the most out of their ballet training during their eight years at the school. There are many ballet schools all over the world and most countries in Europe have their own main one and then lots of secondary ones like they do in England. The aim of these ballet schools is, of course, to train people to be professional dancers and it is fully expected that when you join and train there ballet is your life and soul.