There are actually 2 ways to perform Age Standardisations:
1. Performing a Age Standardisation against the candidate cohort - this will give a typical profile with a mean of 100 and the expected statistical Std deviations.
2. Calculating an Age Standardisation where the test paper has been calibatrated previously against a statistical National sample. In this case because only bright children tend to enter the tests the mean is skewed to the right and is well above 100.
NFER use a technique calling trialling to Age Standardise particular test papers against a National sample.
Most 11+ exams use the former method but Kent 11+ exams uses the latter method. Bucks uses a bespoke hybrid approach which results in a test mean (average) of about 111-112 rather than 100.
One of the anomalies of the national sample approach to Age Standardisation is that quite a few candidates achieve the max Age Standardised score of 140 or 141.
You should bear in mind that this doesn't mean that all candidates who are allocated a score of 140 achieve the same raw score (or anything like)
To quote from the QCA report on Key Stage 2 tests (which Age Standarised tables are available):-
Very low and very high age standardised scores are printed in the tables as ***. This means that they would be below 70 or above 140, but cannot be calculated with the necessary degree of statistical reliability. If an exact score is needed, for example, to calculate an average for the class, 69 or 141 should be used for these pupils.
The same applies to 11+ Age Standardisation, so if 2 candidates each scored 140 this doesn't means that they both scored the same, it just means that they scored so highly they were not able to accurately calculate an age standardised score so they just allocated 140 as the highest avalable. So it's perfectly possible in theory that two children of the same age achieving raw scores of say 75/80 or 80/80 respectively could each get a standardised score of 140! Of course this would depend of the degree of difficulty of the particular test.
Hope this clarifies matters.