I am a learning needs assessor. Using Reading Age is not necessarily the most accurate method or determining performance as it only gives a measure of attainment against their own previous level. Using a standardised reading test would track both progress against a “normal” group but would also give an indication as to a pupil’s performance against their standardised age group. You will then have more than an “opinion” of progress you will have numbers to compare.
I use the GORT5 reading test which give a standardised result and also enables you to undertake miscue analysis to help identify where a child is making mistakes.
It is about £275 for the kit which give you parallel tests to enable retesting every 6 months or so.