Join Date: May 2009
Location: Winnipeg, Manitoba, Canada
You have to consider their skeletal structure to make a good determination. Leg joints typically don't fuse before 2.5 to 3 years old. These are the joints you have to worry about if the pressure is to much for her and she blows into a bucking fit - slamming down hard on those front legs with your added weight can be extremely dangerous to fragile joints.
The back and neck won't finish completely fusing until at LEAST 6 years old and that's about as young as it happens. Usually between 3 and 5 the back is strong enough that you're not going to do any damage. Horses fuse from the legs up and from the tail forward. The neck will be the last thing to fuse, which is why it's considered not wise to let a horse "fight being tied" before they're a true adult because they can seriously damage their neck.
History has shown that horses being started by around 4 usually fare the best in terms of not being injured due to stress on unfused joints. By this age, you KNOW the legs are done, and are as solid as they're going to get, and it's likely the back has fused enough that even if they jump a bit, you're not going to cause damage.
Based on that information, most people think 2 is too young, however it CAN be done safely, as history has also shown that not every 2 year old horse breaks down by 10. Quite the opposite actually - as long as they're started slowly and not worked hard until 3 to 4 years old, it's fairly unlikely you'll cause any damage.
At the end f the day, it really has to be your own informed decision as to when you think is right for you horse! Sometimes it all just boils down to mentality - are they mature enough to handle this or still acting like a big baby? Good luck!
I hope God tells her to smash her computer with a sledgehammer.