Hiya ALJANOBI, welcome to BCF!
I think I have forgotten some of the books I have read, but then discover
that I have just put the memory of it in a drawer in my mind. All it needs
is a nudge for me to remember it all again!
Regarding teenage readers, I think if they get a hook then they are away!
That's where books such as Harry Potter, Twillight series, The Dark Materials
really come into their own.
If someone hasn't caught the reading bug by their teens, it is these types of books
that bridge the gap between children's and adult categories, and allows
a young person a way into reading that is acceptable and comfortable.