4:12 Let no one despise you for your youth. The negative commands here and in v. 14 may indicate that Timothy had a tendency towards shyness or timidity. Furthermore, some in the church at Ephesus may not have accepted his authority (Introduction: Date and Occasion). Timothy was probably in his thirties, and therefore was younger than many of the Christians (and elders) at Ephesus.
example . . . in purity. Timothy is to establish his authority, not by flaunting it but by setting an example of godly living (Titus 2:7).