If the Yeul in A Dying World 700 AF is 15 and Noel is 18?
I thought about this for a while and concluded that it’s possible Squeenix was lazy with changing Noel’s appearance. Yeul might have been older than Noel, and after she died he spent three years wandering around waiting for the portal to Valhalla to appear.
A long shot, perhaps, but possible. It would explain the despair that Noel hints at when he’s relaying his past. Noel is very, very strong; I would think it would take quite a while for hopelessness to claim him.