Okay, I can’t prove to you it was such for “thousands” of years. Would hundreds of years make you happier?
You don’t even know how they were done for hundreds of years. You have “many maiselach.”
And I’ll note that having the parents agree at the age of 3 is NOT what the chassidim are doing now today, so are you suggesting beshows, or are you suggesting agreeing at the age of 3?