God uses people of all ages. He just needs obedience. Noah built an ark at 600 years old. What a great time to begin a new adventure. Sarah was 90 when she had her first child; Abraham was 100. I will admit that David was most likely just a teenager when he "took out" Goliath.
Jesus began his ministry at age 30. Is that young or is that old? Of course, he was killed most likely by those who were older than him because he "bucked" their tradition. Yes, Jesus being fully God is different than us, but in a sense, he didn't even have time to reach his prime. Of course, in another sense, Jesus being sinless was in his prime all the time.
Even as I write this, I have just thought that maybe our sinfulness steals us from experiencing our prime. Jesus lived the abundant life all the time. He was in his prime. Jesus offers us the abundant life, but our sin robs us of that. Our sin keeps us trapped from experiencing our prime. In other words, being in our prime has nothing to do with age, it has everything to do with being connected with God through Jesus Christ. It comes through obedience to Jesus - becoming Christlike. As our obedient relationship with Jesus grows stronger, our sinful habits grow weaker and fade away. As they do, we will more and more reach our prime. We will more and more experience the abundant life.
Yes, my age is now a prime number. Hopefully and prayerfully, through the work of the Holy Spirit, I will more and more be in the prime of my life.