My little guy rode it the first time at 12 months and he's ridden it on every trip since. (He's been 6 times now at age 4.) There's no requirement of any kind for this ride.
No height requirement that I remember, but my 2 year old didn't like it much, it was loud. She's fearless, in general, but did not like the dark rides in the World. She didn't even like the thunder on the Land ride.
I took my 2½ & 4-year-old sons on Maelstrom and they loved it. In fact, we went during Illuminations and rode the ride 4x in a row (we just skipped the film at the end). The CMs were laughing by the 3rd time but there was literally no one else there! It was fun.
Malestrom does not have a height requirement, but my 6yr old niece found it rather scary! It is rather dark at first, and the viking with the eye that lights up didn't help much. When we got off she said that she was never going on again!
If I remember the correctly Maelstrom had some freaky trolls popping out of the water and a huge anorexic looking Polar bear roaring down from his hind legs...THAT to me is more of an issue than the hills and such!! That troll guy freaked me out!!
DS has ridden this many times since his first trip at 13mos, and loves it. On our last trip we went on it several times and one of the last times, we were getting ready to exit the boat and ds was saying "Disappear, Disappear!" and the cm was laughing and saying something like "you've never been on this before, have you?"