你的老师说得对，“当我年轻时”习惯上不能说成As I was young。

因为 as 表示时间意为“当……时候”时，它所引导的从句的谓语动词通常是动作动词。如：

He dropped the glass as he stood up. 他站起来时，把杯子摔了。

As I was coming here, I met your brother. 我来这里时碰到了你的弟弟。

As people approach old age their energy may diminish. 人到老时精力可能减退。

As she knitted she kept the ball of wool beside her. 她织毛线时，总把一团毛线放在身边。

注意，as 引导时间状语从句时，其谓语动词通常不能是静态或状态动词。如：

她出生后不久父母双亡，是由姑母抚养大的。

误：Her parents died as she was a baby and she was brought up by her aunt.

正：Her parents died when she was a baby and she was brought up by her aunt.

但是，当 as 用于引导原因状语从句时，从句谓语可以是静态动词或状态动词。如：

As you weren’t there I left a message. 因为你不在那里, 我留了个信儿。