Brennan describes Aru as much smaller in comparison to Edun, this roiling little ball of light. Imri and Luz also both seem very protective of Aru, as though they're a younger sibling.
Anyway, what I'm saying is--is Ayden the youngest of all the gods' mortal forms because Pelor waiting longer to join them there? Or is he the youngest because he always was? Young and curious and eager and so very in love with the world--
This is so neat, actually! And by that same logic, Asmodeus would be one of the eldest gods because Father Milo is an old man. Older and jaded, first to recognize potential danger and first to act against it (valid or not). Protective and possessive and bitter and angry because why wont his siblings just listen to him he's the eldest he knows best!