I started my mare as a year 2 old although she was halfway to 3...I don't think there is anything wrong starting their training at 2 (if they are going to be really tall, I would wait as likely they are behind on growing), but I think anything heavy should wait until 3 or older. I kept the work really light with my mare and we spent the majority of our time doing leg aides, giving to the bit, just basic stuff, we kept sessions to no more than half an hour (when we first started out only 15 mnutes) but I definitely think if you pay attention you can see what the horse can handle mentally as well as physically...I have seen 2 year olds who completely shut off after 15 minutes and they just kind of have a melt down, and ones who thrive on learning and it doesn't stress them so mentally you can push them a bit.
Physically though even at 3, I wouldn't take my mare into the mountains for a trail ride unless we were taking plenty of breaks. I think there is a reason they set competitive trail at 4 years old -because that is when the majority of the horses are developed enough to really work.
Last edited by bellagris; 12-19-2011 at 10:46 AM.