I would like to have to enter a date of birth when registering But I found nothing in the user fields that could be used there

Have you tried the Discourse Cakeday plugin?

登録時に生年月日を入力する必要があります

rishabh 2020 年 8 月 27 日午前 10:16 2

Discourse Cakeday のプラグインを試しましたか？