Different breeds mature at different times but generally most people prefer to wait til 3/4 because this is when the skeleton is fully fused together and therefore strong enough to hold a saddle and human. I personally wouldn't back a 2 year old but I would definitely get him desensitised and used to some equipment. My yearling is already wearing a bridle and going for walks, and when he's 2 I'll teach him to lunge, but he won't get a saddle on til he's 3
I start on all my Clydes at 1 working with leading and mouthing. At the age of around 2 we start to saddle them and work them. 2 and a half is a an okay age for ground work but he is more likely to have vices.