句意是： Tom 和 Jim 的年龄一样大。
这里使用 as ... as 来表示同级的比较，意味"和……一样"。两个as其中第一个as为副词，第二个as为连词。
基本用法就是 as + 形容词/副词 + as + 名词/ 形容词 / 从句
例句：This book is as interesting as the one I read yesterday.
这本书和我昨天读的那本一样有趣。as + 形容词+ as + 名词
You must run as fast as you can.
你必须以你最快的速度跑。as + 形容词+ as + 从句
如果要比较说一个是另一个的几倍，就是 n times as + adj/adv + as + noun
例句：This book is twice as expensive as the one I bought yesterday.
这本书的价钱是我昨天买的那本的两倍。
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