DD does ISTD grades at her studio and once they enter grade 6 they are watched carefully to see when they are suitable to go onto pointe. DD went on after 6 weeks, age 12 (just) and others wait 6 months. The teachers are careful to make sure they are strong enough regardless of age although I haven't seen anyone younger than 12 on pointe. Grade 6 had 30 min pointe lesson per week while anyone not on pointe does work on demi- pointe. This seems really sensible to me.
At summer school this year many of the 12 yr old girls were regularly doing 2 hour pointe lessons which seems a big jump!