When do you think that a horse is ready for a trail ride? I own a 4 year old thoroughbred mare who is 90 days undersaddle, w/t/c/j and has gone to one show. She loves her barn and arena but I want to keep her entertained! She has never raced*.
I've taken a lot of my young horses out on the trails on their 3 or 4th ride. As long as I'm comfortable that they will give their head if I ask and that I have some kind of whoa.
I think a good trail ride following an experience trail horse is great work for a young horse.
If you horse is 90 days under saddle. I wouldn't hesitate to get them out and put them to work on a trail. But YOU need to evaluate your confidence level and whether you feel like you can control the horse.
The person that trained my horse took him on the trail the first week he was there. She feels, and I agree, that it's a great learning tool for them and the more they are exposed to the better horses they will be.