If only the good die young, that must mean that only the nasty live to a ripe old age. Monsieur Gillenormand, Marius' aristocratic granddad, is exhibit A: by the end of this book, he's well into his nineties, but he still has all the energy and sass of someone in his thirties. Unfortunately, he seems to use that energy to be a real pill.
Gillenormand likes to think that he has eternal youth because he was a real ladies' man for most of his life. But the truth is that he:
Was an idiosyncratic old gentleman and most decidedly a man belonging to another age, the complete picture of the somewhat aloof bourgeois of the eighteenth century, wearing his middle-class respectability with all the assurance of a marquis wearing his title. (18.104.22.168)
Gillenormand is a staunch monarchist, meaning that he supports the French king and doesn't want people around him to even mention the word "democracy." We get the idea, though, that he doesn't have any real political beliefs underlying his position; it's just that he doesn't want anyone to take away his money or privilege—including his own grandkid. When Marius defects from the old man's beliefs and embraces revolution, Gillenormand kicks him to the curb, hoping that Marius will be so poor that he'll have no choice but to come crawling back and apologize.
The old man doesn't realize, though, that money isn't everything. Marius is perfectly happy to live on the tiny scraps of money he can make with his interrupted legal education, and he never does come crawling back. Loneliness eventually causes Gillenormand to fall into deep loneliness. As the book tells us, "He had periods of utter dejection, so greatly did he miss Marius. Old people need love as they need sunshine; it is warmth" (22.214.171.124).
But there's a spark of goodness in this crusty old geezer, and that spark is love. He might be angry at Marius, but he does really love the boy. By the end of the book, Gillenormand accepts Marius (and it probably helps that the revolution fails, Marius is badly wounded, and it turns out that his chosen bride is secretly wealthy). The two of them reconcile and Gillenormand gets to be happy again in the final years of his life.
Who knows, though? The guy might live to be a hundred and twenty.