1. 年纪大了 (nián jì dà le): This literally means "age is big," which is a common way to say "old" in Mandarin. It implies that someone is getting older. You can use this phrase for both people and objects.
Example: 他年纪大了，不能再做这种体力活了。(Tā nián jì dà le, bù néng zài zuò zhè zhǒng tǐ lì huó le.) He is getting old, he can't do this kind of physical work anymore.
2. 老了 (lǎo le): This is a more direct way of saying "old" and is commonly used for people.
Example: 我妈妈老了，身体越来越差。(Wǒ māmā lǎo le, shēn tǐ yuè lái yuè chà.) My mother is getting old, her health is getting worse and worse.
3. 年龄大了 (nián líng dà le): This phrase is similar to 年纪大了, but it specifically emphasizes the "age" aspect.
Example: 他年龄大了，应该退休了。(Tā nián líng dà le, yīng gāi tuì xiū le.) He is getting old, he should retire.
4. 岁数大了 (suì shù dà le): This phrase is also similar to the previous two, but it focuses on the "number of years" aspect of age.
Example: 岁数大了，身体越来越差。(Suì shù dà le, shēn tǐ yuè lái yuè chà.) As we get older, our health declines.
The best way to say "old are" in Mandarin will depend on the specific context and the desired meaning.