In Japanese, the English phrase “12 years old” is written as ‘12歳’ or “12才”.

Both “歳” and ‘才’ are read as “sai,” and “歳” is the standard form.

As an exception, since children in elementary education typically learn the character “歳” before “才,” it is sometimes common to write ages as “12才” during the period after learning ‘才’ but before learning “歳.”

reference:

[「年齢」を「年令」、「○歳」を「○才」と書くことがありますが、正式にはどうなのでしょうか？｜漢字文化資料館](https://kanjibunka.com/kanji-faq/jitai/q0165/) \(Article in Japanese\)