I don't think it will so easy that a large number of children get close to full marks, but if it was the case then there is published research that it can invalidate the Age Standardisation process and in particular disadvantage older candidates.
To quote from I P Schagen (the author of the Age Standardisation methodology used):-
The fundamental assumption of the basic standardization model is that the percentile values may be fitted by a set of parallel straight lines as a function of age. There may be circumstances, however, in which this assumption is not valid. For example, if there is a "ceiling effect" then the test is too easy for the age range and the top percentiles are unable to increase with age at the same rate as the lower percentiles.
There are "mathematical" ways around this to compensate however this is not really satisfactory and tests should be designed such that the average raw score is not close to the maximum score.
For info there was such an issue with the Warwickshire 11+ plus test last year and the previous few years) which resulted in much debate about cheating
Hope this helps