NHK has learned that the Japanese government plans to set the minimum age for a third shot of coronavirus vaccines at 18 for now.
The health ministry's expert panel is expected to approve the use of the Pfizer-BioNTech vaccine as booster shots on Wednesday.
The ministry has said boosters will be administered about eight months after people are fully vaccinated.
The rollout is scheduled to start for healthcare workers in December and for the elderly in January before expanding to others.
The minimum age for the first two doses is 12 in Japan.
Ministry officials say they have set the higher age limit because Pfizer has not given sufficient data to assess the third shot's efficacy and safety, as the booster rollout for children under 18 has just begun overseas.
By February, it is likely that some children under 18 will have been fully vaccinated more than eight months before.
The ministry plans to start considering lowering the minimum age once Pfizer provides additional data.