二十歳

From Old Japanese はたち (patati, “twenty items”).

• (Tokyo) はたち [háꜜtàchì] (Atamadaka – [1])^[1][2][3]
• IPA^(key): [ha̠ta̠t͡ɕi]

二十歳(はたち) • (hatachi) 

1. twenty years old

From 二十 (nijū, “twenty”) +‎ 歳 (sai, “year”).

• IPA^(key): [ɲ̟iʑɨsːa̠i]

二(に)十(じゅっ)歳(さい) • (nijussai) 

1. twenty years old

Jōyō kanji recognized only じゅう (jū) and じっ (ji') as readings of 十 until 2010, even though the latter is actually pronounced じゅっ (ju') today and NHK accepted it already in 1966.

From 二十 (nijū, “twenty”) +‎ 歳 (sai, “year”).

• IPA^(key): [ɲ̟iʑɨːsa̠i]

二(に)十(じゅう)歳(さい) • (nijūsai) 

1. twenty years old

• 未成年(みせいねん) (miseinen)
• 小人(しょうにん) (shōnin)
• 大人(おとな) (otona)
• 成人(せいじん) (seijin)
• 成人式(せいじんしき) (seijinshiki)
• 成人(せいじん)の日(ひ) (seijin no hi)
• みそじ (misoji)