At 40 years old, she became level with high schoolers.
She got to the same level as high schoolers when she was 40.
ご質問ありがとうございます。
1番目の回答でlevel withの言い方を紹介しています。他の組み合わせで、She was level with high schoolers when she turned 40.です。
high schoolersはhigh school studentsの省略です。そして、2番目の文末にyears oldを付けることができますが特に要りません。
ご参考いただければ幸いです。
She finally got on the same level as a high school student at forty.
She finally got on the same level as a high school student at forty.
She finally leveled with high school students when she was forty.
一般的に「対等になる」は **be on the same level as ～** や **be equal to ～** が自然です。
例：She is on the same level as her classmates.（彼女はクラスメートと同じレベルだ）
アメリカ合衆国 
