“I'd learned myself by the age of sixteen that just as girls guarded their virginity, boys guarded something less tangible which they called Themselves.”

VI, line 347
Variant translations:
But who is to guard the guards themselves?
Translated by Lewis Evans, in The Satires of Juvenal, Persius, Sulpicia, and Lucilius (1861), p. 51
Who watches the watchmen?
Famous variant used in the graphic novel, Watchmen by Alan Moore and Dave Gibbons.
The original context is that a husband might lock his wife in the house to prevent her adulteries, but she is cunning and will start with the guards; hence, who guards the guards? The phrase has come to be applied broadly to people or organisations acting against dishonesty or corruption, esp. in public life. See Quis custodiet ipsos custodes? at Wikipedia.
Satires, Satire VI

Dare to Dream by One Direction, https://www.goodreads.com/author/quotes/6422638.Niall_Horan

119
Golden Sayings of Epictetus

"Childhood".
Silex Scintillans (1655)
Context: Dear, harmless age! the short, swift span
Where weeping Virtue parts with man;
Where love without lust dwells, and bends
What way we please without self-ends. An age of mysteries! which he
Must live that would God's face see
Which angels guard, and with it play,
Angels! which foul men drive away.

Source: Digital Fortress

The Evolution Tour: Live in Miami
2007, 2008

During a 34 - 8 rout vs. the Patriots on November 17, 1996 CNN, SI.com - NFL - Shannon Sharpe career retrospective - Monday May 17, 2004 10:38PM http://sportsillustrated.cnn.com/2004/football/nfl/05/17/sharpe.retrospective/index.html,

Source: Digital Fortress

Why Do Little Girls?
Song lyrics, Living Room Suite (1978)