Mistakes will always be made in French, and now you can learn from them.
Wrong: Je suis 25 ans
Right: J'ai 25 ans
Explanation: As every French student knows, the French translation of "to be" is être. However, a lot of English expressions with "to be" are equivalent to French expressions with avoir (to have). "To be ___ (years old)" is one of these expressions: "I am 25 (years old)" is not "je suis 25" or "je suis 25 ans," but rather j'ai 25 ans. This is just something you have to memorize, along with j'ai chaud (I'm hot), j'ai faim (I'm hungry), and all the rest.
Note: The word ans (years) is required in French, even though in English you can just say "I'm 25."
Avoir, Être, Faire
Expressions with avoir
Expressions with être