My kids are roller coaster freaks and were riding coasters at all kinds of parks as soon as they were tall enough! And they're tall for their ages. IIRC, my DS rode Screamin' at age 4 and loved it. OTOH, this is the same kid who at 24 months was laughing his head off as we spun at maximum centrifuge speed on the teacups. When my kids were small, for fun, my DH would toss them high into the air, and they would fly up perhaps 2 feet above his hands, then fall back down to him. They loved it!
So in short, we've been 'training' our kids for thrills since they were infants.
In answer to your question, look at the other rides the child has been on and enjoyed. Let the child look at the Screamin' ride (and see the takeoff from the bridge) before going. Let the child make the final decision without any prompting from you.