I think, at least with our kids, their comfort level depended on how we approached riding rollercoasters. Our kids are both speed demons, but our attitude affected their reactions (no big surprise, really).
DS10 didn't go to DL until he was 5 - he gets overstimulated and we didn't think he could handle it, so we babied him. We started him gradually, with Gadget's Go-Coaster, them moved up to BTMRR and Matterhorn. When he was 6, he reluctantly rode Space, with me holding onto him and reassuring him the entire time. He was tall enough to ride just about everything, but terrified of many rides. We seriously considered not taking him to DL at 7, but went anyway, just to see if he'd be okay. Fortunately, peer pressure kicked in, and he begged to go on Screamin' with a 10-year-old friend. He loved it so much he came running out the gate, screaming happily, and slid across the pavement to me on his knees. He now loves the thrill rides, BUT he is still freaked out by some of the theming, particularly the Yeti.
DS2, on the other hand, has been going to DL since he was 3 months old. From then till this summer, he rode all the "tame" rides, first in a front pack and later graduating to sitting on the seat beside me. If he got scared on a ride, I would gently remind him that he'd ridden it many times before, and suggest that he give it another try. He always had the option of NOT going, but we encouraged him to ride. Every time, it worked like a charm and he had fun. Last month, at 38" tall, he rode the Matterhorn 3 times in one day, and begged for more.
That said, I still get scared for DS10 on Splash. Not because the ride is scary, but because there are no restraints and I don't like relying on his ability to hang on!