Let no one delay to study philosophy while he is young, and when he is old let him not become weary of the study; for no man can ever find the time unsuitable or too late to study the health of his soul. And he who asserts either that it is not yet time to philosophize, or that the hour is passed, is like a man who should say that the time is not yet come to be happy, or that it is too late.

[Μήτε νέος τις ὢν μελλέτω φιλοσοφεῖν, μήτε γέρων ὑπάρχων κοπιάτω φιλοσοφῶν: οὔτε γὰρ ἄωρος οὐδείς ἐστιν οὔτε πάρωρος πρὸς τὸ κατὰ ψυχὴν ὑγιαῖνον. ὁ δὲ λέγων ἢ μήπω τοῦ φιλοσοφεῖν ὑπάρχειν ἢ παρεληλυθέναι τὴν ὥραν ὅμοιός ἐστι τῷ λέγοντι πρὸς εὐδαιμονίαν ἢ μήπω παρεῖναι τὴν ὥραν ἢ μηκέτι εἶναι τὴν ὥραν.]

Diogenes Laërtius (fl. 3rd C AD) Greek biographer [Διογένης Λαέρτιος]
Lives and Opinions of Eminent Philosophers, Book 10, “Epicurus,” sec. 122 (3rd C AD) [tr. Yonge (1915), ch. 27]
    (Source)

Letter from Epicurus to Menoeceus. (Source (Greek)). Other translations:

Let no Man that is Young delay the Study of Philosophy, nor when he is Old, be weary of Philosophers. For no Man can be too early, nor no Man past his Time, in what concerns the Health of the Soul. For he that says, 'tis not yet time to study Philosophy, or that he has past his time, is like to him who says, that the time to attain Happiness is past, or is not yet come.
[tr. Kippmax (1696)]

Let no one be slow to seek wisdom when he is young nor weary in the search thereof when he is grown old. For no age is too early or too late for the health of the soul. And to say that the season for studying philosophy has not yet come, or that it is past and gone, is like saying that the season for happiness is not yet or that it is now no more.
[tr. Hicks (Loeb) (1925), sec. 122]

No one is to delay studying philosophy in their youth or to weary of it in old age. For no one is either too young or too old to be healthy of soul. Anyone who says it's either too soon or too late to study philosophy is like someone who says it's too soon or too late for happiness.
[tr. White (2020), sec. 122]