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.