I have never in my life heard of anyone starting to ride a horse before 18 months of age, and I still personally consider that too young.
I don't care if the baby looked like it was built like a brick ****house, her poor spine was in no way shape or form ready to carry a rider. And even though you may not be seeing the ill effects now, this horse May have a slipped back or arthritis by age 5.
I cannot imagine anyone supporting the idea of backing a horse before age one, under any circumstances.
I am glad at least you have said you won't do it again.
This article is a long read, but informative, I thought. Penned by a vet. http://www.equinestudies.org/ranger_..._2008_pdf1.pdf