Diogenes Laertius (3rd Century AD) is the primary source for the surviving complete letters of Epicurus and for biographical and other pertinent information about him:
Translated by Cyril Bailey, Oxford, 1926
Let no one when young delay to study philosophy, nor when he is old grow weary of his study. For no one can come too early or too late to secure the health of his soul. And the man who says that the age for philosophy has either not yet come or has gone by is like the man who says that the age for happiness is not yet come to him, or has passed away. Wherefore both when young and old a man must study philosophy, that as he grows old he may be young in blessings through the grateful recollection of what has been, and that in youth he may be old as well, since he will know no fear of what is to come. We must then meditate on the things that make our happiness, seeing that when that is with us we have all, but when it is absent we do all to win it.
Translated by Cyril Bailey – Oxford, 1926