If you still look 20 now, you will still look good at 100.
If you looked as you did in your 20’s, then you will be fine until you’re 100.
外見 - appearance, the way you look, your looks
あなたの外見 - your appearance, the way you look
20代 - your twenties
20代の人 - people in their 20’s
30代の男性 - Man in his 30’s
外見は20代 - look as if you’re in your 20’s
〜〜のまま - keep 〜〜、stay 〜〜
20だいのまま - stay in your 20’s, keep in your 20’s
Your appearance is still like someone in their 20’s
〜〜でも - even, is also
100歳でも - even 100 years old, 100 years old is also
いいんです - is good, is fine, is okay
100歳でもいいんです - 100 years old is fine, you will be fine at 100.
I wouldn't mind turning 100 if I still looked like I was in my twenties.
If only turning 100 didn't mean I'd look 100.
加筆です。
①見た目が２０代のままなら１００歳になるのも悪くないんだけどね。
I wouldn't mind ～でも構わない
If I still looked like 見た目が～なら
②１００歳になっても、１００歳に見えないならな～。
If only ～だったらな
didn't mean ～という意味にならない ～にならない
こんな逆の言い方でもできます。
お役に立てれば幸いです！
イギリス 
日本 
