We define young horse as a horse between 12 and 36 months of age based on its actual foaling date, not in calendar years.
Remount horse is a horse of its forth and fifth year of life (from 37th month to 60th). However, the training year for a remount horse that was born in winter starts not on a New Year Day, January 1st, its adjusted birthday, but 4 months earlier, in September of its third year of life.
Specialization should really begin when a horse hit its first day of a 6 y.o. birthday, i.e. 60 months old, and has been trained properly and holistically for the previous 4 years: 2 years as young horse, and another 2 years as a remount horse.