I completely DO NOT agree with you.

I started my mare at 5. She hadn't been handled until I got her. Two months of training (introduction to the saddle and bridle took 2 weeks, then riding her gently for the next month), and that was it.

She wasn't difficult nor impossible to train. In fact, I found it easier to train her, at five, than the 3 year old I trained previously. I would rather take an untrained older horse than a ridden 2 year old.