4 Now the word of the Lord came to me, saying, 5 "Before I formed you in the womb I knew you, and before you were born I consecrated you; I appointed you a prophet to the nations."  6 Then I said, "Ah, Lord God! Behold, I do not know how to speak, for I am only a youth."  7 But the Lord said to me, "Do not say, 'I am only a youth'; for to all to whom I send you, you shall go, and whatever I command you, you shall speak.  8 Do not be afraid of them, for I am with you to deliver you, declares the Lord." 

Jeremiah 1:4-8 (ESV)