Where is the source that says that Stiyl looking like an adult was due to art direction mistake?
I remember something about "old" vs. "new", being interpreted wrong...
I think it was on the Toaru Majutsu no Index Wiki, but I'm not sure...
The Toaru Majutsu no Index Wiki does say it, on both Stiyl Magnus and Kanzaki Kaori's design evolution sections, but it doesn't actually give citations for its claims. The actual source is from Haimura Kiyotaka, the novels' illustrator, on his blog. A translation of Stiyl and Kanzaki's sections:
There has not been much change in Stiyl's design from the very beginning. From top down, he wears a cape, a robe, a black shirt, and black pants. By the way, ever since their introduction in Volume 1, the gap between age and appearance in the magic side characters has been much greater than in the science side characters. There is a reason for this. See Kanzaki's rough for more.
（ステイルの項から続き）１巻の本文初稿を読んだとき、はいむらが抱いた印象は、 「…ああ、これは『オトナの理屈』に子供が反抗する話なんだな」なのでした。 つまり、１巻時点の魔術側キャラであるステイルと神裂は、主人公である上条の目線からは、 「子供にとって、嬉しくない建前や理屈を並べる大人」に見えたハズなのです。
Kanzaki's design had a lot too it, so hers gave me the most trouble. At this stage, the cut of her jeans was only partway. (Continued from Stiyl's rough) When I read the first draft of Volume 1, I came away with the impression that it was the story of children rebelling against the reasoning of adults. That is why I thought Kamijou would have seen Stiyl and Kanzaki as adults that used reasonings and principles that a child would refuse to accept. At the time, I never thought Index would continue this long, so I went with my impression of the two and gave them designs that did not really match their ages...and by about Volume 7 I was starting to regret it.