In chapter 16, Janine is fixated on the number 60. What is the significance of this number? Why is she fixated on it and what does it imply for her future?

Janine Roby, soon-to-be-Comeau, has been looking forward to making an entrance at the Fairhaven High School football game. She doesn’t care much for the sport but is thrilled by the idea of ascending the bleachers in her white jeans and halter top. Now in her forties, she’s put a lot of effort into losing the weight she carried as an unpopular teen. She should be enjoying her small revenge on the men who rejected her in high school and the women they chose instead. Instead, all she can think about is the number sixty.
Janine has just learned that her fiancé is sixty. Not fifty, as he’d told her. Not forty, as he looks. She’s upset that he lied: sixty! She’s upset by the way this reflects on her own insecurity: sixty. Their near-decade in age difference was inspirational to Janine. It gave her hope that she would remain youthful and desirable. She would make up for time lost to an unhappy adolescence and an unsatisfying marriage. If Walt can look this good at fifty, she thought, so can she.
She looked forward to “at least twenty years’ worth of spirited, vigorous sex, having largely missed out on the last twenty by being married to Miles.” In twenty years, Janine would be sixty—and Walt would be an octogenarian. He would not be her distinguished Silver Fox, he would be elderly. She isn’t able to enjoy the event she carefully planned: the game, the entrance, the sun on her shoulders, and all eyes on her. Now, she thinks sixty. She’s beginning to see sixty. “How could a man who didn’t look fifty yesterday,” she wonders, “suddenly look sixty today just because of a date printed on a piece of yellowing, folded paper.” She sees the cheerleaders, thin and young and limber, soon becoming old and joyless. She sees them married and unsatisfied, slipping into motorcourts for affairs with men who turn out to be the same as the husbands they cannot stand. She sees her own destiny all around her, and it all feels like the same sentence: sixty, sixty, sixty.