It has become necessary for me to feed her food that is suitable for her age.
It has become necessary for me to feed her age-appropriate food.
She needs food that is appropriate for her age.
年齢に合わせたえさが必要になってきた
It has become necessary for me to feed her age appropriate food.
It has become necessary for me to feed her food that is suitable for her age.
年齢に合わせたえさ=age appropriate food, food that is suitable for her age.
あくまでペットの話なので、ペットを主語として書くよりも、飼い主目線で説明したほうが自然な文章になります。
そのため、
彼女の年齢にふさわしい餌を与えることが必要になってきた、という文章にしました。
もしくは必要をneedとシンプルにとらえると下記の通りになります。
She needs food that is appropriate for her age.
I have to start feeding my cat food that's age appropriate for her because she's getting old.
この場合、次のような言い方ができますよ。
ーI have to start feeding my cat food that's age appropriate for her because she's getting old.
「うちの猫は歳をとってきたので、彼女の年齢に合った餌をあげるようにしないといけない」
food that's age appropriate for xx で「XXの年齢に合った食べ物（ここでは餌）」と言えます。
ご参考まで！
日本 
カナダ 
