じゃないんだから

you're not ..., so ...

→ だ
phrase Rare #23,753
you're not ..., so ... An emphatic, conversational pattern used to remind someone that they are not something and should therefore act accordingly. Often used when scolding or urging someone to behave more maturely.
JA: もう学生じゃないんだから、時間を守りなさい。
EN: You're not a student anymore, so be on time.

もう子どもじゃないんだから

You're not a child anymore.

医者じゃないんだから

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

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.

いなかじゃないんだから

It's not like we're in the countryside.

— 魔女の宅急便 (1989)