Someone who has turned 20 between April 2nd of previous year and Coming of Age Day of the present year.
Someone who has become of age 20 after 2nd April of last year until Coming of Age Day this year.
前年: previous year / last year / the year before
成人の日: coming of age day
2つの日付の間の期間を示すためにbetweenがつかえます
何々日までの場合: until＋日付になります（例: until April 2nd)
20歳になった人：someone who has turned 20 / someone who has become of age 20
日本語文に書いてないですが、英語で書く時present year か this year 書いたらもっと分かりやすいなります、今年の成人の日という意味です。
People who have turned 20 years old between the 2nd of April last year and this years Coming of Age day.
"前年の4/2〜成人の日までに20歳になった人" という日本語は、以下のように英語で説明ができます。
「People who have turned 20 years old between the 2nd of April last year and this years Coming of Age day.」
「20歳になる」=「turn 20 year old」
「成人式」=「Coming of Age day」
I hope this will help you.
お役に立てば嬉しいです
インド 
イギリス 
