Jeremiah 1 and Luke 9 are my favorite passages dealing with this:
The Call of Jeremiah (Jeremiah 1)
4 The word of the Lord came to me, saying,
5 “Before I formed you in the womb I knew you,before you were born I set you apart; I appointed you as a prophet to the nations.
6 “Alas, Sovereign Lord,” I said, “I do not know how to speak; I am too young.”
7 But the Lord said to me, “Do not say, ‘I am too young.’ You must go to everyone I send you to
and say whatever I command you. 8 Do not be afraid of them, for I am with you and will rescue you,” declares the Lord.
9 Then the Lord reached out his hand and touched my mouth and said to me, “I have put my words in your mouth. 10 See, today I appoint you over nations and kingdoms to uproot and tear down, to destroy and overthrow, to build and to plant.”
The Seventy Return
"The seventy returned with joy, saying, "Lord, even the demons are subject to us in Your name." And He said to them, "I was watching Satan fall from heaven like lightning. Behold, I have given you authority to tread on serpents and scorpions, and over all the power of the enemy, and nothing will injure you."...at that very time He rejoiced greatly in the Holy Spirit, and said, "I praise you, O Father, Lord of heaven and earth, that You have hidden these things from the wise and intelligent and have revealed them to infants. Yes, Father, for this way was well-pleasing in Your sight...many prophets and kings wised to see the things which you see, and did not see them, and to hear the things which you hear, and did not hear them." Luke 10:17-19, 21, 24.
My favorite part of this story is Jesus' ecstasy as He thanks God for His abundance and totally slams the devil! His disciples were probably his age and younger, and they ended up being the very first revivalists and church-planters in history! John was the youngest disciple and had the deepest intimacy with Jesus. Jesus is ready and willing to use people of all age. In His kingdom, it is not age that matters, but willingness and obedience! He says that in the last days, old men will dream, young men will have visions, and young women will prophecy. Isn't his Kingdom exciting?!