It's old. I don't know how long it's been there. You shouldn't drink it.
I don't know how old it is. I wouldn't drink that. You might get sick.
1) It's old. I don't know how long it's been there. You shouldn't drink it.
「それ古いよ。そこにいつからあるかわからない。飲まないほうがいいよ。」
2) I don't know how old it is. I wouldn't drink that. You might get sick.
「それどれだけ古いかわかんない。私なら飲まないな。気持ち悪くなるかもよ。」
「いつのものかわからない」は
I don't know how long it's been there.
I don't know how old it is.
と表現できます。
ご参考になれば幸いです!