Even though he looks an adult, he's still a kid inside.
simさん
ご質問どうもありがとうございます。
様々な表現があると思いますが、いくつか自然な表現を紹介します。
1. Even though he looks an adult, he's still a kid inside.
2. Even though she looks an adult, she hasn't yet reached the corresponding level of maturity.
3. Mike looks an adult, but in every other way he is still a big baby really.
4. Even though I'm an adult now, I still don't feel like it.
5. Even though I'm an adult now, I still don't feel like I've grown this much mentally.
語彙:
- corresponding = 該当する
- level of maturity = 「大人的さ」のレベル
- I don't feel like it = ～であることを感じない。
- to grow mentally = メンタル的に成長する
お好みに合わせて使い分けてみてください。
ご参考にしていただければ幸いです。
Even though his exterior is a developed adult, his inside is a kid.
You may get big on the outside, on the inside, you're still not an adult.
Even though his exterior is a developed adult,「外見は成長した大人ですが」 his inside is a kid.「中身はまだ子供」
You may get big on the outside,「君は大きいだけど」 on the inside,「中身は」 you're still not an adult.「まだまだ大人じゃない」
Even though people grow up physically, they don't grow all that much inside.
この場合、次のような言い方ができますよ。
ーEven though people grow up physically, they don't grow all that much inside.
「人は肉体的に成長しても、中身はそんなに成長しない」
ここの「大人になる」は to grow up physically のように言うと自然で良いかと思います。
don't grow all that much inside で「中身はあまり成長しない」と言えます。
ご参考まで！
ドイツ 
オーストラリア 
カナダ 
