I think it’s completely uncontroversial that younger humans are incapable of reasoning, no? The only area for discussion is at which age (if any) this changes for an individual.

I would say that's a pretty controversial take unless you strictly mean babies. At the point a kid can walk they are certainly capable of some level of reasoning because just walking successfully prior to it becoming muscle memory requires a fair amount of reasoning and children very rapidly figure out new things both spatial and logical.