My son is starting in year 8 in Slough Grammar, and I am glad the 11+ are over for my family.
His school choice was the same as yours.
For both Burnham and Slough grammars, the pupil age is taken into account, but it depends on the cohort. It is probably 5 + points, but it is just a guess.
This is explained here
http://www.elevenplusexams.co.uk/standa ... scores.php
and discussed in several places on the forum, such as here:
Anyway, 90% is a good score, even for a September birthday. Is this for all three subjetcs?
Also, you need to make sure that you have been tested on papers that are of the same level as the exams. What have you used so far?