Hospital visit rules

I want to know everything about hospital visit rules in Japan, but the thing I want to know the most is who can't visit patients at the hospital. Kids of what ages are not allowed to visit? Can family members and friends that are younger than the allowed age be expected?

That would depend on the hospital, the ward, and the ailment.

Most (if not all) hospitals have restrictions on the hours, the number of visitors at once.

Depending on the ward, some hospitals do not allow elementary school kids 6 - 12 (or younger), or junior high school kids 13 - 15 (or younger), or under 18 (because they spend a lot of time each day in classes together - though i expect this to apply only for at very restrictive hospitals or for specific wards where they want to take extra care with any infectious ailment).
If I have to guess about hospitals in general, Ifd say 12 years old or younger (elementary school kids) might not be allowed.

But please do ask at the hospital where your family member/friend is staying at.

Visiting rules are often written on the hospital's official website. And rules change from time to time. For example, during the pandemic, a lot of hospitals didn't allow visitors at all.