What is maturity? Seneca, Epistulae 1.4.1-2
Seneca encourages Lucilius to correct and compose his mind, mastering his fear of things that don't matter or even exist. You can listen to this passage <here>.
Persevera ut coepisti et quantum potes propera, quo diutius frui emendato animo et composito possis. Frueris quidem etiam dum emendas, etiam dum componis: alia tamen illa voluptas est quae percipitur ex contemplatione mentis ab omni labe purae et splendidae. Tenes utique memoria quantum senseris gaudium cum praetexta posita sumpsisti virilem togam et in forum deductus es: maius expecta cum puerilem animum deposueris et te in viros philosophia transscripserit. Adhuc enim non pueritia sed, quod est gravius, puerilitas remanet; et hoc quidem peior est, quod auctoritatem habemus senum, vitia puerorum, nec puerorum tantum sed infantum: illi levia, hi falsa formidant, nos utraque.
Continue as you have begun, and make what haste you can, that you may sooner enjoy the fruits of a mind corrected and composed. You will enjoy some of these fruits even as you are still mending and composing. Nevertheless, the pleasure of witnessing your own mind shine bright, unstained by any blot, is a special one. Doubtless you recall the joy you felt as you put away the fringed toga (†), donned the toga of a man, and were led into to the public square. Expect even greater joy when you put away your boyish mind, and philosophy inscribes you in her list, among the men. Immaturity is no longer a concern for you, but childishness remains, and it offers a more serious threat—worse, really, because we cling to childish and even infantile vices with the authority of old men. Boys fear things that don't matter. Infants fear things that aren't real. We fear both.
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(†) Freeborn Roman boys, and some girls, wore a white toga with a purple stripe on its fringes. When they came of age as adults, the boys replaced this with a plain white toga (unless they happened to be curule magistrates, mourners at a funeral, or otherwise exempt from normal custom).