It absolutely depends on the CM. We went last fall when my little guy was just over 40" (at home), and he was able to ride Splash Mountain, Tower of Terror, Test Track, BMTRR, etc several times. But there was one time that he was denied at Splash and one time he was denied at ToT. If they can fit a paper between their head and the measuring stick (at least these particular CMs), then it's a no-go. In fact, the ToT time that he couldn't ride the CM at the entrance let him in, and we waited in line 25 mins, got to the top, and the CM at the start of the ride measured him and wouldn't let him ride. This was after he'd already ridden it once before. It was kind of a crap shoot. Looking forward to later this year when he's clearly tall enough.