The number of suicides is the highest among men in their 40s and 50s.
ご質問ありがとうございます。
"The number of suicides is the highest"＝「自殺者の人数で一番多いです」
"among men in their 40s and 50s."＝「40代と50代の男性の中で。」
Example in context: "I thought that women were more likely to commit suicide, but actually the number of suicides is the highest among men in their 40s and 50s."
ご参考になると良いです。
In Japan, men in their 40s and 50s have the highest rate of suicides.
おっしゃられている内容は、以下のようにも表現できると思いました(*^_^*)
In Japan, men in their 40s and 50s have the highest rate of suicides.
「日本では、40代とか50代の男性は最も高い自殺率を持っている」
rate「割合」
以上です。
少しでも参考になれば幸いです(*^_^*)
アメリカ合衆国 
日本 
