I agree with saanj H, you can learn whenever you want if you have the determination. Full immersion is ideal, or at least frequent talking with someone fluent in the language you are trying to speak.
That being said, the younger you are, the easier it is to learn a language. When you are a young child, your brain is pre-dis positioned to learn a new language. When you are a teenager you have more time on your hands.
As you get older, you typically have less time to spend learning another language.