frivolous

a. 轻佻的,妄动的,琐碎的

She thought that reading romantic novels was a frivolous way of spending her time.

她认为看浪漫小说是虚度时光。

At 18, he's still rather frivolous and needs to grow up.

他到了十八岁仍然很不懂事，很不成熟。