I'm sure mtlion has seen a lot more boots that I have, so I'm encouraged that his observations match mine from a smaller sample.
I've worked with groups of kids of very similar size (height & weight) with bigger differences in foot sizes, and corresponding differences in boot sizes. Nothing I've seen would indicate an intrinsic difference in stiffness due to the size of the boot (Langes included).
Having said that, every boot has simple adjustments with buckles & power straps that can change the working flex to better suit the skier. The overall quality of the fit (see factors mentioned by mtlion) will influence how successful simple buckle adjustments will be in optimizing flex.
Given what you can do with buckles & power straps, this also means that kids who haven't spent some time getting their buckles done up optimally may have created flex problems for themselves. This can be too stiff or too soft.
Chanwmr, I'm wondering what you saw as an indication that different sizes had different flex, and following the OP, stiffer flex for larger boots?