For kids, the structure of it is perfect. Lots of repetition and small incremental steps of increasing difficulty.
For an adult, it is incredebly boring and there are more fun ways to learn music.
With my kids, I have found that any sooner than 4 is a waste of time. I'd rather do something else with them like play house, teach them connect 4 or checkers, or just listen to their stories as we doodle with crayons to be more rewarding with those precious few hours before bed. Sitting at a piano telling them to sit straight, concentrate, fingers on the keys, do it again, and again, and again until they cry just before bed just sucks. It would take a 1 or 2 year old a few weeks what a four year old can do in a few hours. And by the time they are 5, would be at the same level of skill anyway. So why bother having to go through that torture when it would be so much enjoyable when their brains are more developed?