also: れいさい
noun
under 1 year old; 0 years old
Refers to a baby who has not yet reached their first birthday. Often used in medical, developmental, or statistical contexts.
０歳の赤ちゃんは予防接種を受ける必要がある。
Babies under 1 year old need to receive vaccinations.
この保育園は０歳児から預かっています。
This daycare accepts children from 0 years old.
Rare kanji spelling; reading れいさい is less common than ゼロさい.
Compound of ゼロ (zero) or れい (zero) and 歳/才 (years of age). The reading ゼロさい is more common in modern Japanese.