The best survey data I've found for this information is linked here [1]. It breaks data down by age range for those who read at least one book in the prior year. It defines avid readers as having read 50 books/year but doesn't do an age breakdown for this subset.

1 - National Endowment for the Arts - "U.S. Patterns of Arts Participation: A Full Report from the 2017 Survey of Public Participation in the Arts" - <https://www.arts.gov/sites/default/files/US_Patterns_of_Arts...>, page 44