《牛津英语用法指南》第三版指出：

请注意 be + ... age 这一结构（不用介词）。

When I was your age I was working. 我像你这么大的时候，就在工作了。（不能说：When I was at your age ...）

At your age I already had a job. 我在你这个年纪已经有了一份工作。

所以，下列句子正误判断是对的：

正：At your age I had the burden of a large family on my shoulders. 

正：When I was your age I had the burden of a large family on my shoulders. 

误：When I was at your age I had the burden of a large family on my shoulders.