Let's break it down. to indulge - To yield to the desires and whims of, especially to an excessive degree mockery - insulting or contemptuous action or speech; a subject of laughter, derision, or sport So "indulgent mockery" means their smile is making fun of the young people without holding back, perhaps imprudently so. The writer sounds really bitter about these elders and is using this passage to describe them to you while insulting them to make himself feel less hurt by them.

Indulget = here: self-indulgent=self-satisfied, self-pleased. Mockery = way of making fun of someone else, an act of teasing someone else. An act of teasing someone else while being self-indulged by the teasing.

While I would agree agree as to what indulgent mockery means, I would disagree with whom they are mocking. Indulgent mockery implies a certain self-satisfaction when you are seeing someone make a fool of themselves. When you see your drunk brother-in-law flirt with a girl you happen to know likes only other women, you might break into an indulgent smirk. The indulgent ones in the passage above aim their mockery at other elders who, unlike them, have not come to accept that they are no longer young. They laugh internally at the others' attempts to keep up with the youths in things like fashion, pop culture, social relevance, knowing that eventually, they too will have to accept that they're "getting too old for that shit". So, I guess I would also disagree with the notion that indulgent mockery, as it is used in the passage, is used in the spirit of spite and self-preservation. These people are wise and don't typically resort to mockery. This is why he used the word indulgent, because they felt a little justified in giving in to that form of pettiness. Indulgent is used in the same context as when you don't normally drink but your daughter just got married and offers you a glass of champagne. You will indulge on this occasion because the situation warrants it.

Wow, you are quite ambitious! This is some pretty hard core literature a lot of native English speakers would have some trouble with. The language is quite flowery and a little dated. I had to think about this for a second because I have a gut feeling of what "indulgent mockery" means, but it was difficult to pin it down precisely. Keep in mind there are a lot of words here like "trod" (past tense of "to tread"), "wantons," "chastened," and "scorn" that you'll probably never use unless you are writing a book yourself.