The thirtieth year seems to be a very significant time in people's lives - both in the Old and New Testaments.
Joseph was 30 when he became ruler in Egypt.
David was 30 when he became king.
Ezekiel was 30 when he began his ministry. Ezekiel must have submitted to Jeremiah, in his younger days. He must have listened to Jeremiah's prophecies and studied them as a youth. God Who saw the faithfulness of this young man, decided that Ezekiel would be a prophet and not a priest.
Jesus was 30 when He began His earthly ministry, and the Father spoke these words about Him publicly from heaven, "This is my beloved Son in Whom I am well-pleased." (Mt. 3:17).
Most of the apostles were around 30 when they began their ministry.
Even today, it is probably around that age that God wants to begin to lead his children into the specific ministry that He has for them. But prior to that date, God has to spend many years in preparing us for that specific ministry.
If you surrender to God totally and allow Him to prepare you during your teens and twenties, then you can be ready by the time you are 30 (or 35), for that specific ministry that God has planned for you.
But a lot of young people are impatient and unwilling to wait. I am not saying that you cannot go out and serve God before you are 30 years old. You can start serving God even when you are 16. But in your early years, God has to keep you under authority in order to guide you and protect you. But many young people chafe under such submission to authority and, as a result, are never broken and prepared for the ministry that God has planned for them.
Even Jesus needed that training to submit to Joseph and Mary for 30 years before He entered into His ministry; how much more we?