Let no one despise you for your youth, but set the believers an example in speech, in conduct, in love, in faith, in purity. -- 1 Timothy 4:12
I think this verse is often misunderstood. We read it very often to say that we shouldn't let anyone look down on us for being young. This is undoubtedly true, but it is just the tip of the iceberg of what Paul is getting at, and indeed, most people who bring the verse to mind stop after the first admonition: "Let no one despise you for your youth."
When we take the whole verse into consideration we realize that Paul is telling us young'uns how to prevent the looking down upon our youthfulness. In these days of arrested development and adultolescence -- an epidemic among young men -- it is often justifiable that our youthfulness is despised. A generation of manboys slouches into inconsequence, video game controllers in hand, and 1 Timothy 4:12 is no covering for such idiocy. Neither is it a cover for cage-phase doctrinal arrogance, lustful self-indulgence, drooling gluttony of shiny gadgets, loutish chauvinism, and outright laziness.
The second admonition -- "set the believers an example in speech, in conduct, in love, in faith, in purity" -- tells us how to effect the first. What Paul is telling Timothy, rather, is that the key way you guard against people despising your youth is by growing up. 1 Corinthians 13:11 and all that.
Rejoice, O young man, in your youth, and let your heart cheer you in the days of your youth. Walk in the ways of your heart and the sight of your eyes. But know that for all these things God will bring you into judgment. -- Ecclesiastes 11:9