Capacity first
A stated maximum is only a first screen. It does not explain which ages, beds, meals, or rates are allowed.
Before you reserve
Does this Japan hotel room accept your whole family?
Screen the exact room against the full party, child ages, stated capacity, bed-sharing language, extra-bed needs, and connecting-room requirements. Then send the unresolved questions to the property.
Bed sharing is property-specific
There is no safe universal age rule to apply across every Japanese hotel, room, rate, and booking channel.
Keep written confirmation
Ask about the exact room type and save the answer with the reservation before the free-cancellation deadline.