Reading ages are not the same as Reading attainment. Reading ages are derived from the average raw or scale scores at different age points. The red line in the chart shows the average scale scores e.g. the average scale score for 8 year old students is 272. Therefore, any student with a scale score of 272 will have a Reading age of 8 years. For example, a 12 year old student with a scale score of 276 and in the bottom 10th percentile will have an age-equivalent close to 8 years (as shown in the black lines below).
The year on year average growth in Reading ability is much higher at younger ages (e.g. average increase from 228 to 272 between ages 7 and 8 years). For older students the year on year growth is much smaller over ages 13 years (e.g. average increase from 368 to 376 between ages 14 to 15 years). Therefore, a Reading age of 17 years is unlikely to be that much different from a Reading age of 14 years. In most cases it is not sensible to relate scores for students with above average ability to Reading age as Reading ages relate to an average. In this example, the most we can say about a 12 year old student with a scale score of 400 and in the top 10th percentile is that their Reading age is higher than 15 years (refer to the orange line). Therefore, there are issues with using Reading ages especially with above average students and it is best to use standard age scores (SAS) for measuring progress or monitor trends.