Here the minimum age for a Level I distance ride (25-39K) is 5 years old. If you have been doing endurance for ten years then I am sure you must be familiar with the rules and regulations of the sport. LSD conditioning is great for a younger horse who has the ground and saddle training basis that makes them safe to ride on a trail. A few years of the correct trail work can prepare them physically and mentally for the stresses of a distance competition. You should check out a great website for lots of valuable information on the sport of endurance.