To use an analogy in human sports, professional figure skaters and ballet dancers typically start training around age five. If you’re lucky, you have teachers, coaches, trainers – who are cognizant of the needs of young people’s growing bodies, need for the right nourishment, and not overtraining young muscles. Overtraining leads to early burnout, permanent physical damage, and possibly health problems later in life. This principle applies to both human athletes and horses.
In the extreme case, this filly died – and she shouldn’t have. Human athletes have died, too early, under similar conditions.
Ekaterina wrote their story in her memoir My Sergei. Both she and Sergei began their skating training at the age of 5, and were paired in their teens by the elite Soviet Army Central Sports Club [CSKA, pronounced “SESS-ka”.]
Their coaches and choreographers were wise enough to not work them in the wrong ways, that they went into burnout. Athletes who go into burnout are often forced to retire. (I mention ballet here because it’s an integral part of training to be a figure skater, because many of the skills are the same.)