Well, my daughter went on it when it first opened and she was almost four - the ads on at the time made it seem like a perfect "first time" roller coaster for a pre-schooler. And Disney describes it as "small drops," but because of the way the carts "swing" on an axis, her little head was bobbing back and forth like crazy - we have the PhotoPass video footage of it. She still won't ride it, even when we can manage FastPasses for it (sigh) and she's eight now. She tried the Barnstormer the next year, which also has small drops, and loved it - still rides it every chance she can get. We finally got her to try Big Thunder Mountain Railroad a few days ago (also described as small drops), and she was okay with it. I think it was the "swaying" of 7DMT that really didn't agree with her.
Of course, it really depends on the pre-schooler. I know a three-year-old who went on Splash Mountain this past spring with no problem, and that has a "big drop." I think he'd have no problem with 7DMT. But my eight-year-old was so miffed when she got off of it (combo of the big drop and getting soaked in the front seat) that she guilted us into getting her a big stuffed Brer Rabbit in the gift shop on the way out.