To become a professional ballet dancer you should start at the age of 7 but no later then 11. A pre-professional ballet student studies an a major ballet academy affiliated with a ballet company like SAB (for NYCB) or JKO (for ABT.) They take 20-30 hours of class a week and by age 17 they are asked to apprentice with the company. After a year they are asked to join the corps de ballet, or are let go. Another option would be to study at a major ballet academy that is not affiliated with a company such as the Harid Conservatory or CPYB. You then audition for the ballet company's school at age 16 and learn their repertoire for a year before being made an apprentice.Hope that helped
Any age from 2 to about 10 if you want to be a proper proffesional because I started at age 2 and a half and I have done grade 1 grade2 grade 3 grade 4 grade 5 grade 6 and I am halfway through advance and trying to get ready for intermediate I think that is what it is called)
Around about 9, but you can start before or after. Also you have to go to a really good school and train basically every day of the week.