I started to feel a decline in my body and mind around the age of 40.
Once I turned 40, I started to notice a weakening in my physical and mental well-being.
この場合、次のような言い方ができますよ。
ーI started to feel a decline in my body and mind around the age of 40.
「40歳ごろから心身の衰えを感じ始めた」
a decline in my body and mind で「心身の衰え」と言えます。
ーOnce I turned 40, I started to notice a weakening in my physical and mental well-being.
「ひとたび40歳になると、心身の健康の衰えに気がつくようになった」
a weakening in my physical and mental well-being で「心身の健康の衰え」と言えます。
ご参考まで！
カナダ 
