I know what defy means; but since I'm trying to correctly translate that phrase, I want to know which sense of this verb I should be looking at.
I couldn't figure it out from here.
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The explanation in your citation is fairly straightforward.
Suppose X looks far younger than his or her age. We might say
“The way X looks utterly defies age.
The phrase represents a personification of aging as a powerful person whose commands X has refused to obey.
The phrase can also be used in the context of a person who behaves in a way that is admired in the relatively young, but is unusual in those who are older.
He is over 50, but still is playing professional football: no one has defied age as he has.
So the phrase you are asking about indicates some youthful attribute in someone that is quite unusual given the person’s chronological age.