This manga describe quite different about the reincarnation matter. Even if you have the mind of an adult, baby body has to affect your mind somehow.
For sure! Brain development makes a huge difference. Children are physically incapable of regulating their emotions. The part of the brain that makes that possible doesn't develope until kids are much older.
Has to do with the prefrontal cortex I think. Maybe 5-8 years old or something? It's been quite a while since I did research on this stuff.
Honestly the character should also be effectively blind for a while. The parts of the brain that do visual processing are also not developed for some months after birth. The would have visual input, but without there hardware to process it, their vision would probably be pretty similar to being on LSD or mushrooms. A lot of the visual effects of hallucinogens are from disabling the brains ability to recognize the edge of things, for example. All that stuff is done with hardware, like a GPU, and it takes a while fot it to get built and calibrated.
Brains are fascinating.