In my experience most horses are not completely physically mature until they are about 8 years old. Warmbloods are VERY slow maturing horses.
Yes, a horse should begin being ridden at about 3 years old, but the first year or two should just be about allowing the horse to become familiar with the basic concepts of walk, trot, canter, with the weight of the rider. Under no circumstances should the horse’s head carriage be altered during this phase. While the horse is becoming accustomed to carrying a rider he must be allowed to use his neck & head to counterbalance this new weight.
As time goes by the horse will let the rider know when he is ready for harder work because these basics will be solid foundation. When this happens the gymnastic work of developing the horse’s pushing power can begin. This period generally takes between 6 months & 1 year. Once the horse is capable of pushing himself through space carrying the rider the trainer can begin asking the horse to learn collection and begin movements.
At all times the trainer must observe the concept that the development of the horse’s gaits is always the top priority, along with maintenance of proper rhythm at all times.