reading age 
noun
A child’s reading ability expressed with reference to an average age at which a comparable ability is found: twenty per cent of this year’s new entrants at 11 had reading ages of 16-plus his reading age will never be more than 8 or 9...
- Their reading level of English is too low, with an average reading age of eight years.
- The people we deal with have lost their hearing at a young age or were born deaf and the average reading age of such a person is eight years old.
- ‘The average reading age of most of our parents is between five and nine years,’ says McGow.