At least in the show-verse, Daeron is def the oldest of Daeron II's grandchildren. I think it goes:
Daeron
Valarr
Aerion
Matarys
Aelor & Aelora
Aemon
Daella
Egg
Rhae
Daenora
I think Daeron is about 2-3 years older than Valarr, and Aerion is only around a year younger than Valarr. Matarys came the year after Aerion. I think Aelor and Aelora were born before Aemon, who was born in 198 (confirmed), and then we also know that due to Egg being born in 200, that means Daella was born in 199. Rhae is probably only 1-2 years younger than Egg. The real wildcard is Daenora, who could've been born roughly around the same time as Rhae or much later when everyone would've been in their teens, which is sad to think about as she would have no siblings or cousins around her age; but that unfortunately happens with kids born later in their specific generation of their family (poor girl probably spent her whole childhood wishing she had cousins to play with, only to then be wed to Aerion).
