Please look at the sentence below. Thanks
(Ella is a girl for 18. ) Is this sentence right? Is it equal to (Ella is 18 years old.)
 Thank you very much.

"Ella is a girl for 18" is not correct.

You can say "Ella is a girl of 18 (years old)", but it is extremely formal/old fashioned.

Here's an example: 

"We have a boy of 8 years old that is about 4 feet tall (1m20). Will he be allowed on the water slides?"