Euripidis Tragoediae (Nauck 3rd ed.)

...it is unknown to no one that he had to mold it; nor would you be wrong if you traced back to this training in particular that passion for philosophizing [that was] fixed in the mind of Euripides, for which he himself owed the nickname of "the stage philosopher" 16. At what age he first descended into the tragic contest can hardly be determined with certainty, given such a great disagreement among witnesses 17. However, it is credible that the poet’s first tragedies were received by the people with moderate favor and that he experienced harsher judgments for some time before he was announced victor in the first place and gained for himself that favor of his contemporaries by which he flourished among later generations. Nor did his father Mnesarchides see the answer of Apollo fulfilled 18. Finally, after the most severe overturning of all things, which invaded the state of the Athenians as the Peloponnesian War encroached, the citizens seem to have lent a more receptive ear to Euripidean wisdom 19: what the causes of this were, will be indicated below. Nevertheless, we have received that the first prizes were awarded to Euripides only five times 20. And the poet...

...for the writings of Heraclitus the "Dark One" were read by Euripides, [which] is not at all improbable. Cf. Tatian, Oration to the Greeks ch. 3 and Diogenes Laertius 2, 22.

16) Cf. Athenaeus IV, p. 158 E and XIII, p. 561 A; Sextus Empiricus, p. 666, 1; Clement of Alexandria, Stromata V, p. 688; Origen, Against Celsus 4, 77, p. 215; Eusebius, Preparation for the Gospel X, p. 504 C; Vitruvius VIII, pref. § 1.
17) Life of Euripides vs. 30: "He began to teach in the archonship of Callias, in the eighty-first Olympiad, the first year. He first taught the Peliades, when he also came in third." In the same place, shortly before vs. 20: "He began to compete having reached twenty-six years [of age]." Where "twenty-five" should be written, suspecting [the text] in comparison with Thomas Magister p. 139, 21: "He began the contest concerning these things having turned twenty-five." Differing from these is Aulus Gellius (15, 20, 4): "He began to write tragedy at eighteen years old." In the Parian Marble (C.I. 2374, 75) these things are contained: "Euripides, at [43] years of age, first won with a tragedy — in the archonship of Diphilus at Athens (Olympiad 84, 3 or the year 441 B.C.)." The fabrication produced by Suidas, "He turned to tragedy upon seeing Anaxagoras suffering dangers for the dogmas he introduced," we consider sufficient to have touched upon by a word.
18) Plutarch, Moralia p. 496 F: "Nor did the fathers [of Euripides and Sophocles] know of their victories."
19) Certainly in Olympiad 87, 4 (428 B.C.) he was announced the first victor; cf. Argument to Euripides’ Hippolytus.
20) Aulus Gellius, Attic Nights 17, 4, 3: "Marcus Varro says that Euripides, although he wrote seventy-five tragedies, won in only five [of them], while he was often defeated by some very lazy poets." It is clear that the poet won not with tragedies, but with five tetralogies. Suidas: "He took five victories, four while living, and one after death, when his nephew Euripides exhibited the drama." Vitiated in Life of Euripides vs. 131, "he holds 15 victories" is handed down, which writing error was propagated by Thomas Magister p. 139, 20. For the rest, what the individual victories of Euripides were, cannot be explored by our resources; cf. Bergk in Meineke's Comicorum Graecorum Fragmenta, vol. 2, p. 904.