Age could have an impact on achievement tests like your state testing and Iowas, depending on which norms they choose to report. State tests are almost certainly normed by grade, without consideration given to age of student. While the CogAT should be scored by age norms, the achievement tests are grade-based.
The range of difference you report, however, is considerably larger than what one could reasonably ascribe solely to differences in age. You are describing what appears to be a child who is relatively 8-9 months older compared to their norm group than the second child, but performs more than a year ahead academically. Though conceivably this could be the combined effects of siblings typically being within 10 IQ points of each other (so say 110 vs 120) and age, with the higher ability sib being a little older, and the other being a little younger, which might possibly create just enough of a synergy such that the first scores in the GT ID range, while the other scores in the average range (which is what 35th %ile is).
I think we haven't seen enough data to interpret these results with a high degree of confidence.
