As long as the vet okays it I don't think it would be awful, but personally I would be hesitant. My old horse was a TB and had been a brood mare for years, and produced 5 awesome foals that did well on the track. We are good friends with her old owners and he called us when she was 20 asking if he could breed her one last time. We decided not to let him because we were riding and showing and doing well, and let me say I am happy we didn't. A year later when she was 21 her kidneys failed and she sadly passed away. Yes horses can and do live well into their late 20's early 30's, but they don't always make it long, you just never know.