1Twenty years old. Usually read with the special kun reading 'hatachi'; in the forms with 歳/才, it can also be read 'nijussai' or 'nijissai'. Culturally significant in Japan as an age associated with adulthood and the coming-of-age ceremony.
彼女は今年はたちになる。
She turns twenty this year.
二十歳になると日本ではお酒が飲めるようになる。
In Japan, you can legally drink alcohol once you turn twenty.
成人式は二十歳を祝う大切な行事だ。
The coming-of-age ceremony is an important event celebrating turning twenty.
二十歳の誕生日に家族で食事に行った。
I went out to dinner with my family on my twentieth birthday.
2(archaic) An old word for the number twenty itself. Sometimes written 廿. Essentially unused in modern Japanese; encountered mainly in classical texts or old documents.