Okay, first of all. The angle of the shot with Ienzo has Ven up close to the viewer, so it's entirely possible that Ienzo's not as short as he looks and may, in fact, be closer to Ven's height.
That being said, he may be 14 or 15. The experiments happen a year later.
Let's look at Riku's ridiculous growth spurt between CoM and KH2.
Maybe Ienzo was the same and that's all there is to it. Aging in KH is weird, and even Sora and Kairi had strange growth spurts within a year (Kairi and Namine gained like a head or two).
That being said, he may be 14 or 15. The experiments happen a year later.
Let's look at Riku's ridiculous growth spurt between CoM and KH2.
Maybe Ienzo was the same and that's all there is to it. Aging in KH is weird, and even Sora and Kairi had strange growth spurts within a year (Kairi and Namine gained like a head or two).