also: れいさい
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.
Babies under 1 year old need to receive vaccinations.
This daycare accepts children from 0 years old.
新生児 specifically means a newborn (first 28 days), whereas ０歳 covers the entire first year.
Standard modern spelling using Arabic numeral and 歳.
Common simplified variant using 才 instead of 歳.
Formal or official style using the kanji numeral 〇.
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.