It is, in absolute terms, harder for a 9 or 10 year old to score 160 than for a 5 year old, because the expectations are higher for an older child. In normative terms, however, the likelihood of a child scoring 160 is the same at every age, since standard scores are based on the percentage of the population that is able to attain a particular raw score. It is not all that unexpected, however, for a high-achieving five year old to display slightly inflated normative (standard) scores, due to the presence in the norm group of children who have had no exposure whatsoever to academic skills. IOW, because the majority of 5 year olds have negligible academic skills, having even a little bit of skill rapidly pushes an individual up the normative ladder.
For reading comprehension, that subtest is not normed for 5 year olds, therefore it is not possible to obtain any kind of normative score.
In the grade 4 or 5 reading comprehension item sets (which would be about age 9 or 10), a student would have to receive full credit for all items to obtain a maximum (160) standard score. At exactly 10 years old, a student could miss ten items, and still receive a 160 standard score.
Assuming the scores were validly obtained, there is no data-based reason to believe that the standard scores and percentiles you have obtained for your children are not "true" compared with the nationally-representative standardization sample. However, the national norms may not represent your specific community.
No, it is not possible to receive a standard score above 160 on the WIAT-III.
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I just realized that you may well be talking about the WIAT-II (UK/AUS/NZ), in which case the specific normative information does not apply. However, the rest of the discussion does.
