The Church teaches that it is an intrinsically and  gravely disordered action since it is using sex outside of it's purpose, which is in marriage. According to the Church (and to Our Creator) the purpose of sex is procreative and unitive and to be engaged in within a covenantal marriage. The rest of us must be completely chaste.  
I think you way over-exaggerate how telling a 15 year old that masturbation is wrong will harm them psychosexually. As to being punishable" by God in the fires of hell for all eternity"; masturbation and all sex acts outside of God's plan and purpose for humanity is gravely sinful, but it is not up to you or me or even the Pope to determine a person's culpability in the matter. It is for God.