Miss Anne de Bourgh is about Darcy's age. So, 28. I had for some reason remembered Lady Catherine saying "since SHE was in HER cradle we planned the match," but it's "since THEY were in THEIR cradles we planned the match." And Darcy has been destined for his cousin "from his earliest hours." I had thought before that Lady Catherine told herself that Darcy hadn't proposed to Anne because Anne had been too young, and that when Lady Catherine said "now, at the moment when the wishes of both sisters would be accomplished in their marriage" yada yada the "now" was because Anne had just turned twenty-one or something. Or even 25, and maybe she wouldn't be able to inherit Rosings if she married before then. But, no. She's Darcy's age. And I've never heard of 28 being the kind of age 21, 25, or even 30 have historically been treated as by law and culture.
Lady Catherine is certainly not rational about this, but I can't conceive of any way for her to be THIS deluded, or for that "now" to make sense. Unless Darcy was... at Oxford until he was 27? (Which would be a whole other kettle of weird.) Or maybe he was traveling Europe?