My son (also tall for his age) was 6 the first time he went on RnR -- although he was not by himself, and I don't think he'd have wanted to do it if I hadn't been right there next to him. He liked it, although the loud soundtrack threw him a bit (I'd educated him about the upside-down parts, the fact that it would be dark, etc., but apparently forgot to mention the blaring music), and he commented later that, "those Smith guys' music is awful!". Still, he much preferred it to Expedition Everest (he didn't like the backward portion) or Space Mountain (very rough) -- RnR was the only one of the three coasters that he wanted to do again when we made our next WDW visit a couple of years later.