不成熟 Englisch:

He’s very immature.

He often behaves like a child - he's so immature.

My brother is an immature boy.

My acquaintance's almost twenty but his behavior is a bit immature.

Grow up – stop being so immature.

He’s nearly 26 now but he’s still so immature.

He behaves in a very immature way.

She's rather immature for her age, don't you think?

He's so immature. He acts like a child.

1. If I'm immature, you're prenatal. / 2. Because you're immature, irresponsible, reckless, empty-headed and impulsive. / 3. And emotionally immature, totally dependent upon external validation.

1. If I'm immature, you're prenatal. / 2. Because you're immature, irresponsible, reckless, empty-headed and impulsive.