Thank you very much, everybody!

The sentence should be: "In eight weeks the reading age of all children was improved by six months". In schools, when they test a child's reading skills they often measure this in years and months. For example a child may be six years old but have a reading age of four years (i.e. their reading skills are at the level of an average four-year-old child). If the reading age of all children was improved by six months, then this would suggest that their reading age would have increased 6 months from their previous reading age.

Personally I think perhaps the text is supposed to read "In eight weeks time the reading age of all children was improved by six months". That would make perfect sense. Maybe a typing error?

I'm not sure about this specific passage, but it may have something to do with the way reading levels are measured in school. For example, when I was in school (New York, US), we would take a test to evaluate our reading level. This sounds like a class average, since each person would receive a whole number assessment with no fractions (up to Level 12, which is senior high school level). I can imagine that the class average went from, say, 11.2 to 11.7 after eight weeks of this unspecified "method." The 0.5 improvement would represent roughly a semester, or six months in reading level.