I am exactly the same way. Was a voracious & fast reader throughout schooling up until college age. Officially diagnosed ADHD in college when the material became hard enough for me to not "brute force through with my raw intelligence anymore" (psychiatrist words, not mine) so I was likely masking all the way up through high school. Younger brother was diagnosed at a more typical age which made my late diagnosis easier.

I wonder if it has to do with the mental burden we start to accumulate in transitioning to adulthood. Much easier to hyperfocus on books when we don't have the weight of finances, careers, complex relationships, etc.