Foreigners often make some strange mistakes in Chinese when they study the most difficult language--- ---Chinese. We try to analyze typical sentences to correct these errors.

          Māmɑ bǐ wǔ nián qián jiù duō le.
False: 妈妈比五年前旧多了。
          Māmɑ bǐ wǔ nián qián lǎo duō le.
True: 妈妈比五年前老多了。

英语可以把“旧”和“老”都议成old，但是汉语的“旧”是用来指事物经过长时间或经过使用而发生变化。原句说的是妈妈年岁大了，不能用“旧”，要用“老”。
旧 and 老 are all translated into "old" in English. But 旧 in Chinese only indicates that things have changed after being used for a long time. The present sentence speaks of advancement in age, thus 老 rather than 旧 should be used.