Humbert is sexually obsessed with youth in Lolita. He doesn't care that he is aging; in fact he never mentions that. He is obsessed with "nymphets," young girls aged nine to fourteen. He does not hesitate to mention that once girls get past that age they are no longer attractive to him. College-age girls are way out of his age-range. Sure, Humbert recognizes all of the unrefined silliness and consumer vulgarity of youth, but that's a small price to pay. Sitting on a park bench watching young girls play hopscotch and skip rope, Humbert is in bliss: "Let them play around me forever. Never grow up" (1.5.11).
Humbert reads youth almost exclusively through a sexual lens, struggling to see that innocence as anything other than a quality to exploit.
Humbert's disgust of youth sits in a strange relation to his love of it.