じゃないんだから

it's not like …

→ だ
phrase Rare #22,380
it's not like … Often used to remind or scold someone, with the unspoken implication that they should act accordingly.
JA: もう子供じゃないんだから、自分で決めて。
EN: You’re not a child anymore, so decide for yourself.

もう子どもじゃないんだから。

You're not a child anymore.

子供じゃないんだから、いい加減にしろ!

You're not a child, so grow up already!

もう子供じゃないんだから。

You're not a child anymore.

医者じゃないんだからさ。

It's not like you're a doctor.

子供じゃないんだから。

You're not a child, you know.

もう子供じゃないんだから、自分のことは自分でやりなさい。

You're not a child anymore, so take care of yourself.

田舎じゃないんだから

It's not like we're out in the sticks.

— 魔女の宅急便 (1989)

なんて言っちゃダメよ人は見かけじゃないんだからね

You shouldn't say things like that. You can't judge people by their appearance.

— 崖の上のポニョ (2008)