Thank you both for the clarifications.
It seems to me as though there are many factors at play here: Bodies in general have changed over the years, with average heights, foot length, and average age of puberty has changed drastically, even in the past few decades. In 1910, average height for a woman was 5'1 with a shoe size UK 2/US 5, but now, the average woman is 5' 6", with an average shoe size of UK 7/US 9. Average onset of puberty was 16.6 yrs in the late 1800s, and now it is around 10.6 years.
Layer on top of that how much Ballet as an art form (and gymnastics, and track, and basketball, etc.) has evolved into being much more taxing on the average body, and we can see how much more important it is to take into consideration whether an activity is safe at any given moment in a child's development.