18 months is WAY to young for a horse to be carrying the weight of a person, it's like asking a 10 year old to run in a marathon.
I don't even like riding 2 year olds, and would rather wait until the horse is atleast 3 years old; unfortunately alot of people like to train that early. I'd rather do the training and know it's done right, than have someone else ruin the horse. Even then, I will not work a 2 year old hard...w\t and basically working on the basics of being ridden. I can work them a little more on the ground, rather than making them carry weight for harder work at that age. And if a horse isn't built well enough at that age, I will not even think about getting on until he can mature more.