Honestly, I feel like some of the really extreme examples fit pretty well with the fan theory that Kyubey selects their "target demographic" less because of the reasoning that they give (ie., teenage girls have unusually strong emotions) and more because someone like Madoka made a wish that significantly limited who they could make a contract with in a way that was retroactive (with their rationale being a post hoc way of making sense of a restriction that feels voluntary to them, but that they don't actually understand very well).

They seem to have a very, very broad interpretation of what constitutes a child as opposed to an adolescent. Even ignoring Yuma's example, because frankly her appearance and backstory makes it seem like she might just be very malnourished and emotionally stunted, no reasonable person would treat Tsuruno as being in the same "developmental stage" as even Kako or Sana (and then Kako, as young as she is, is at a drastically different point in her life than Ui even if their age difference is only 2 or 3 years). Kyubey seems to be going by some strictly biological standard that has nothing to do with psychological development or emotional maturity, which makes the most sense if they're trying to cast a broad net by working through loopholes, rather than trying to select ideal candidates based off of their personality and/or emotional range.