Be 5/10/20 vs. Be 5/10/20 years old

[Jupiter]

Hi teachers,

Happy Volentine's Day to you all :D

Acoording to Lonman Essential Activator, 1999, be 5/10/20 etc is used only about people and be 5/10/20 years old is used about people or things.

My question is what should we use about animal?
Eg. My dog is 3 or my dog is 3 years old?

Thanks in advance

Jupiter

[Conchita]

You can say it both ways:

- My dog is 3.

- My dog is 3 years old.