Jamieson, Fausset and Brown on 1 Timothy 4:12

12. Let no man despise thy youth--Act so as to be respected in spite of thy youth (1Co 16:11; Tit 2:15); compare "youthful" as to Timothy (2Ti 2:22). He was but a mere youth when he joined Paul (Ac 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" (1Ti 5:1), and generally in respect to his duties in rebuking, exhorting, and ordaining (1Ti 3:1), which ordinarily accord best with an elderly person (1Ti 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 (Ga 5:6). The oldest manuscripts omit, "in spirit."

in purity--simplicity of holy motive followed out in consistency of holy action [Alford] (1Ti 5:22; 2Co 6:6; Jas 3:17; 4:8; 1Pe 1:22).