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We have ages for most of them
11: Leaf, Selene, Elaine
12: Kris, May, Dawn, Lyra
14: Hilda, Rosa
15: Akari
16/17: Serena
Red is stated to be 11 in the Gen I manual so presumably Leaf is too. By extension, Chase and Elaine are probably 11.
Starting the island trial at 11 is a recurring line in the Alola games, the number is used because it's a Kanto reference since it's the anniversary Gen, so Elio and Selene are 11.
Brendan/May and Lucas/Dawn are listed as being 12 inside their design documents, and Brendan/May are outright said to be 12 in ORAS. While I never saw documents saying the exact ages for the Johto protagonists, people seem to categorize them as also being 12, and their heights/weights would support it. Kris would be in line with Ethan and Lyra.
Hilbert and Hilda were initially envisioned as being 16, but their ages were settled as being 14. Nate and Rosa are presumably the same, as Hugh is listed in the character document as being 14.
Rei and Akari are pointed out to look around 15 or so in PLA.
Calem and Serena are said to be the same age as Tierno, who was listed in the XY character concept document as being 16. While information definitely has been altered since then, Emma (who is 16) speaks to the protagonist like they're older than her, so 16/17 is the approximate age.
I haven't seen ages listed for Victor/Gloria or Florian/Julianna, but the former look around 14 to me and the latter around 11/12.
