Thanks for the training tip A knack for horses :) I will defenlty do that.
And all her heart is in to it, she enjoys it so much :P she's actually doing pretty good and she has no sign of pain what so ever.
If she has no major health issues that are going to put in her pain from the gaming, as long as she enjoys it I see no problem starting your 15 year old on barrels for the fun and local shoes.
Have a good time with it! Just take care of her and make sure she is always good condition, because she is starting to get up in age. Take plenty of time to warm her up and cool her down, and of course, keep her in shape.