Using ans and age . when i form a sentence j'ai qunize ans. The direct translation is i am 15. How do i say"I am fifteen years old"?

The standard and most common way is to say J'ai 15 ans in both cases.

"ans" relates to age in the sentence format you are using.

Je suis 15 ans. 

I     Am.   15. Years old

I think once you say,  'J'ai qunize ans,' you will have included age already - 'ans' means; years old in English. 

Hi Dank

You have an important question. When you say I am 15 years, it's understood that you are saying "I am 15 years old". Don't get stuck on it, every language have certain sentences where something or the other is not said or written but it is understood. 

So, forget adding age in the sentence, it is okay, and keep on learning further. 

why is it " tu as" used some times and vous avez other times