FACTS
-Small breasted women exist. Flat chest doesnt mean prepubescent girl.
-"Selene" have small breats, but still uses bra, so isn't a little girl.
-They wear such clothes because Alola is fucking hot (this is said in Nintendo Treehouse), and these clothes obviously make them look young (customization does wonders, just compare Serena's clothes).
-Calem and Serena were over 16 years old (because Emma were 16 acording to Xerosic data and refers us like olders).
-SM website says Hau has same age as protags.
-Hau's body is same size as the Calem and Serena's. Yes, it is. Just see pic related, poke ball are huge in Brendan/May hands (12 years old), but are so small in Calem and Serena hands. If you arent blind, you can see that its the same case with Hau and his poke ball. So Hau body size = Calem body size
-Sequel protagonist have same age as his predecessor (Gold 11 like Red, Nate 14 like Hilbert)

Conclusion
-Hau, "Elio" and "Selene" actually were 17 years old, like Calem and Serena.


Red and Blue = 11 years old (Pokemon Red and Blue game manual)
Ethan and Lyra in = 11 years old (Red/Blue's age)
Brendan and May = 12 years old (Pokenav Plus info)
Lucas and Dawn = 11/10 years old? (Pokedex reveals that they are smaller than Ethan/Lyra)
Hilbert and Hilda = 14 years old (Famitsu info)
Nate and Rosa = 14 years old (Hilbert/Hilda's age)
Calem and Serena = 17 years old (deduction according to Xerosic data)
"Elio" and "Selene" = 17 years old (deduction according Red-Ethan and Hilber-Nate relation and Hau's body)