Formal:
* Tengo [age] años. (I am [age] years old.)
* Tengo [age] años de edad. (I am [age] years old.)
Informal:
* Tengo [age]. (I am [age].)
* [age] años. ([age] years old.)
* Voy a cumplir [age] años. (I'm going to turn [age] years old.)
You can also answer in a humorous way:
* No te lo voy a decir. (I'm not going to tell you.)
* Tengo la edad suficiente para saberlo. (I'm old enough to know it.)
* ¿Cuántos años te gustaría que tuviera? (How old would you like me to be?)
Remember:
* Use the formal response when speaking to someone older or in a formal setting.
* Use the informal response with friends, family, or in casual settings.
Let me know your age and the level of formality you need, and I can give you a more specific response.