The birds were in immature plumage.

这些鸟儿羽翅未全。

She is emotionally immature.

她情感上不成熟。

...grossly immature drivers who flout the rules of the road.

无视交通法规、极端鲁莽的司机

She's just being childish and immature.

她在耍小孩子脾气，真是任性。