Don’t let anyone look down on you because you are young, but set an example for the believers in speech, in conduct, in love, in faith and in purity. 1 Tim. 4:12
Timothy faces an uphill battle. Think of the questions he's likely to hear:
"Who is this new guy in town, again? Why should we listen to him?"
"I'm old enough to be his father and he's going to tell me I've been misunderstanding the Scriptures all my life?"
"Yes, our scholars have misread the prophets for centuries, but you've managed to figure it all out. Did your Gentile dad help you with that?"
So Paul encourages Timothy. "If they're going to doubt you because of your age, make sure they can't doubt the godly maturity that's evident in your conduct." Timothy is to continue preaching and teaching, despite the challenges he faced (1 Tim. 4:13-14), while adorning his teaching with unsurpassed character.
Be diligent in these matters; give yourself wholly to them, so that everyone may see your progress. Watch your life and doctrine closely. Persevere in them, because if you do, you will save both yourself and your hearers. 1 Tim. 4:15-16
Though Timothy had been placed in a position of leadership within the young Ephesian church, he still has some rough edges that are going to be apparent to those he leads. There aren't any issues that would disqualify him from planting this church, but it is going to take some effort to grow into a seasoned leader - diligent, watchful, persevering effort.
In just a handful of verses, Paul prescribes the following commands for Timothy in his role as a teacher: devote yourself, do not neglect, be diligent, give yourself wholly, watch closely, persevere. Why all the fuss? Paul's letter contains several warnings about false teachers, and sadly, those who haven't followed Paul's prescription have fallen away from the faith (1 Tim. 1:3, 1:20, 4:1-3, 6:3-5, 6:20-21). Timothy's faithfulness and perseverance in the diligent study of God's Word is tightly bound to his own faithfulness and perseverance in the faith as well as the faithfulness and perseverance of those he leads.
Teaching is serious business and requires serious commitment. Not something to be taken lightly, feeding Jesus' sheep is hard work. Quickly reading some texts can lead to confusion or misunderstanding (John 5:39-40; 2 Pet. 3:15-16), but patient, thorough study and application yields fruit - "you will save both yourself and your hearers." Repeatedly, Paul stresses the importance of holding fast to scriptural truth:
Do your best to present yourself to God as one approved, a worker who does not need to be ashamed and who correctly handles the word of truth. 2 Tim. 2:15
Studying scripture doesn't require a Ph.D. in New Testament Greek, but we should always be careful to work diligently at acquiring a correct understanding and application of what we've read for ourselves or heard from others (Acts 17:11).