1 Timothy 4:12

Let no man despise thy youth--Act so as to be respected in spite of thy youth (Co1 16:11; Tit 2:15); compare "youthful" as to Timothy (Ti2 2:22). He was but a mere youth when he joined Paul (Act 16:1-3). Eleven years had elapsed since then to the time subsequent to Paul's first imprisonment. He was, therefore, still young; especially in comparison with Paul, whose place he was filling; also in relation to elderly presbyters whom he should "entreat as a father" (Ti1 5:1), and generally in respect to his duties in rebuking, exhorting, and ordaining (Ti1 3:1), which ordinarily accord best with an elderly person (Ti1 5:19). be thou an example--Greek, "become a pattern" (Tit 2:7); the true way of making men not to despise (slight, or disregard) thy youth. in word--in all that thou sayest in public and private. conversation--that is, "behavior" the Old English sense of the word. in charity . . . faith--the two cardinal principles of the Christian (Gal 5:6). The oldest manuscripts omit, "in spirit." in purity--simplicity of holy motive followed out in consistency of holy action [ALFORD] (Ti1 5:22; Co2 6:6; Jam 3:17; Jam 4:8; Pe1 1:22).

— Jamieson-Fausset-Brown Commentary (public domain)