From Epicurus' "Letter to Menoeceus":
Let no young man delay the study of philosophy, and let no old man become weary of it; for it is never too early nor too late to care for the well-being of the soul. The man who says that the season for this study has not yet come or is already past is like the man who says it is too early or too late for happiness. Therefore, both the young and the old should study philosophy, the former so that as he grows old he may still retain the happiness of youth in his pleasant memories of the past, the latter so that although he is old he may at the same time be young by virtue of his fearlessness of the future. We must therefore study the means of securing happiness, since if we have it we have everything, but if we lack it we do everything in order to gain it.
I do significantly less philosophy these days than I once did and I never styled myself a philosopher or even an advanced student of philosophy, but I did appreciate this introduction in Epicurus' letter to Menoeceus.