My daughter's teacher allowed students to go en pointe when;
1. they had the technique...what's the purpose of getting them up there if they can't do anything?
2. they had the strength and physique...if they were overweight or didn't have muscle strength, it could cause foot or leg damage.
3. they had started entering puberty, an indication that their bones were maturing, and less likely to be injured on a growth plate.
All three criteria had to be met, UNLESS a dancer was unusually strong and talented. No student, despite ability, was allowed en pointe until age 11 or older.