Children don't complete their set of 20 baby teeth until they are two to three years old.

孩子到两三岁时才长齐20颗乳牙。

When I was eight years old, my sister was three years old.

我八岁的时候，我妹妹三岁。

Her son is three years old.

她儿子三岁了。

The boy was only three years old.

男孩只有三岁大。

I was three years old.

我才三岁。