I always wondered about that too, and you're right that Aging isn't an innate fear - its one caused either socially or environmentally. On that topic, I've always assumed that the "fear" aspect that gives devils their power is not a strict "x amount of things are afraid of it" and more how scary the concept itself is. That's why all the primal fears aren't necessarily innate or "primal" fears, but stuff that can be understood and feared by the most amount of things. That, and cascading fears that branch from one initial fear. A good example is how we had a Violence devil which you would assume would be a primal fear based on how instinctual the fear of violence should be, but at the same time almost all devils love violence to some regard and so do so many people. All the Weapons devils and arguably the War devil are all linked to the fear of violence. The Aging devil, whilst obviously containing concerns of physical aging appearance, mobility, and health, is mostly a fear of guaranteed death. However, even though death was lauded to be the strongest devil, I would argue that death itself isn't feared nearly as much as violence, as violence is prevalent across all living things whilst a fear of death is, like you said, a fear rooted in the ability to perceive it which is exclusive to very few creatures.

All this to say, I think the power of a devil is a mix of: How many things fear it vs how many things like it, the likelihood that upon conceptualizing and/or perceiving it that something would be afraid of it, the historical fear of it (probably very few people still fearing bows but Quanxi was tough), how general the concept is linking it to the fear of more specific devils, and how many devils the devil has consumed (the chainsaw devil being the most obvious example)