In the pseudonymous letters of Ignatius, there is one letter to him. The author is Mary of Cassobelae. I don't know who she is either, but she must have been converted to the faith under Ignatius's ministry.
She exalts young people who are faithful to the Lord in their youth. She gives young ages to people in the Bible that I don't know how she got the young ages. She thinks that Solomon was 12 years old when he decided which mother had the baby. What in Scripture makes her think he was 12? I assume he was at least in his thirties when he made this decision. He was young when David made him king in his dying hours, but he I don't think he was 12 even then.
Daniel was indeed young when he was taken to Babylon and refused to eat their meat. He was an old man when they threw him to the lions' den.
Jeremiah was still young when God called him to prophecy, in fact, he wasn't even born. He started when Josiah was king and finished when the Babylonian exile happened thirty years later.
David was probably 17 when he killed Goliath, but I can't prove that either.
But just the same, it is good to follow God at any age. It is also better to still be following him when you are old and have not been lured away by the world.