I think I would echo what the previous poster has said, if your DS or DD want to go on the ride and are the right height, let them. If they don't, don't make them go on it, simple as that, let them choose. My youngest DS chickened out right at the top of the queue sorry line for Space Mountain, aged 5 or 7, cannot remember which time it was. It is a bit daunting to go on a rollercoaster by yourself but I think it is the Mams (Moms) that worry about the kids more, than the kids worrying about going on by themselves.
With the RnR, we tried it for the first time in 2000, the lads were 9 and 11 then. The 9 year old didn't like the fact he fell on to his harness when we went upside down because he was a small person. He still remembers it, five years later, but it is still keen to go on it again especially after we said he has grown so he will feel the seat out more. The other lad went on the Kumba at Busch Gardens at 9 and loved it, though he had to prise his fingers off the hand bar! It very much depends on the kid themselves and I suppose by going on gentler rollercoasters may give an indication on what kind of child you have.