How we cite our quotes: (Canto.Stanza)
Quote #1
And will not love dare to trust itself in truth,
And Love is taught hypocrisy from youth (1.72)
Byron thinks of youth as one of the only times in life (maybe the only time in life) when a person can be genuine. From that point on, the adult world gets a hold of you and pulls you into its realm of hypocrisy and double-dealing. This is all a shame because it prevents people from ever knowing genuine love and makes them obsessed with popularity and appearances.
Quote #2
His youth and features favoured the disguise,
And should you ask how she, a Sultan's bride,
Could risk or compass such strange phantasies,
This I must leave sultanas to decide. (1.115)
Once Don Juan dresses up like a woman, he's able to fool most people because he's so young it's hard to tell he's a man. Being a younger boy means he doesn't have facial hair and his features are still smooth, so the point Byron is making here is that, the younger a person is, the more difficult it is to tell his or her gender.
Quote #3
It is enough that Fortune found him flush
Of Youth, and Vigour, Beauty, and those things
Which for an instant clip Enjoyment's wings. (10.5)
Don Juan is chockfull of youth and vigor, and these qualities often make other people jealous of him. To hear Byron tell the story, you'd swear that adults base 90% of their actions on the fact that they are jealous of those who are younger and more beautiful than them.