When our son was nine, he went in line with us for BTMRR, but bailed at the last minute. It's very interesting, because he does have a issues with heights, but LOVES water slides.
At 10, he finally tried both BTMRR and Splash Mountain. While still unsure about Splash after his first time, he loved BTMRR. He did decide to try Splash a second time and liked it after that.
He even tried Tower of Terror that year. I doubt he'll ever do that one again.
I should mention that Disney helped me get over some of my height issues. I used to not want to go on inverted coasters, for example, but I have no problem with them now that I have tried RnRC. I hated my first time on Everest and Splash, but love both now. Splash is actually probably one of my top 5 rides in WDW, now. BTMRR never really bothered me, though.