There are two main forms of deceit that the devil uses to lead young people away from virtue.
The first of these is to make them think that serving the Lord consists in a life of melancholy, where there is no enjoyment or fun. That is not true, my dear young people. I want to teach you a method of Christian life that is happy and contented, showing you what true enjoyment and fun is, so that you can repeat the words of the prophet David: we serve the Lord in holy joy, we serve Domino in laetitia (The Lord in happiness)...
The other deceit is to hope for a long life, where conversion comes in old age, or at the point of death... Who can guarantee that we will live to grow old? That would be making a pact with death to wait for us, while life and death are in the hands of the Lord. If God were to grant you a long life, listen to what he tells you: the path that a child takes in his youth is the one he takes in old age, until death... If we begin living a good life while we are young, we will remain good while we get old, and we will have a good death and the beginning of eternal happiness.
Saint John Bosco
Opere Edite/2
LAS, Roma 1976, pp. 185-188