Both of my daughters were 4' tall at age 7. DS was one inch short at age 7, but is tall enough for RnR this year (at 8)and is very excited. I tried to send him out of the room when the news story came on, but he recognized the ride right away and still wants to ride. We talked about how sometimes people have things wrong with them that they don't know about and the rides make it worse.
I don't believe that kids don't have the ability to say "no" to rides. I have seen lots of scared kids not wanting to go on rides. All my nieces are like that. Now if they are that scared and the parents still make them go, that's another story. My own kids are not afraid of anything and I only have seen tantrums when the line is too long for us to wait or when they were still too little to ride.
Robin M.