こんにちは！ペンパルの子に成人式のことを教えたいんですが、英語で何といったらいいのか分かりません。知っている人がいたらぜひ教えて下さい!!

私も以前知りたくて調べました。
たしか、Coming−of−age ceremonyだったと思います。 　

英語の紹介文を見つけました。。。

January 15 is Seijin-no-hi. It was established as a national holiday in 1948. On this day, those who have turned 20 are blessed and a coming-of-age ceremony is held. Most women attend this ceremony in a colorful, long-sleeved kimono.

In Japan, on turning 20, one is recognized as an adult. The age of 20 is an important juncture when one is granted full rights as a citizen, beginning with the right to vote. Drinking and smoking are also approved from this age.