我母亲和他母亲一样苍老翻译成英语

as old as有两层意思，一是一样苍老，二是同龄。

如果要区分，建议翻译成：

My mother looks as old as his mother.
Or
My mother and his mother are both old and grey.

上面的加了just非常好。

My mother is as old as his.

my mom is just as old as his..

My mother is as old as his(mother).