They absolutely do have the strength to seriously injure and kill themselves - young necks are so fragile, and you get 200lb or so flinging around at the end of one at the wrong angle, which doesn’t take much, and it’s broken.
The process of teaching to tie starts from about Day 1 when you start the process of teaching them to lead. A horse who TRULY leads - isn’t just following - pretty much automatically ties because he had truly learned to move such that pressure is released. Getting there is the problem for some people 
I’d never tie a horse, no matter the age, without him knowing how to give to pretty on the lead rope from all directions, and to my hand on his poll.
Granted, my direct experience teaching youngsters to tie is limited to 2, but I’ve also taught several uneducated adults to tie, and the process is just the same.
My weanling was tied in his stall for grooming and hoof picking. He graduated to a straight tie outside his stall, and then to cross ties. The rope was always such that it was in easy reach for me and could be undone by me but not him.
Not truly getting the giving to pressure, and learning to move forward, is where most people set the horse up for failure, IMHO. Most horses WILL startle and maybe hit the end of the ties at some point, and maybe even somewhat “forget” how to move forward into that again, but if they truly know how to move forward at your request, you kick-start their brain again and all is good.