>Red: 11 years old (RBYFRLG) (http://www.gamesdatabase.org/Media/SYSTEM/Nintendo_Game_Boy//Manual/formated/Pokemon_-_Blue_Version_-_1998_-_Nintendo.pdf), 14 (GSCHGSS)(Pokémon Gold, Silver, and Crystal take place 3 years after the Generation I core series games), 28 (B2W2)(Pokémon Black 2 and White 2 take place 17 years after the Generation I core series games), 30 (SMUSUM)(Pokémon Sun, Moon, Ultra Sun, and Ultra Moon take place 19 years after the Generation I core series games)

>Brendan/May: 12 (as rival) (ORAS) (Pokémon Omega Ruby and Alpha Sapphire: Brendan/May, 12, a Pokémon Trainer from Littleroot Town. - Hoenn News Network, emphasis added)

>Hilbert/Hilda: 14 (http://raim2005.blog18.fc2.com/blog-entry-980.html Site is in Japanese)

>Elio/Selene: 11 (SMUSUM) (Wicke asks if the player and Hau are 11, the minimum age required to take on the Island Challenge, and the player can respond with "Yeah, I am" or "How'd you know?".)

>Rei/Akari: 15 (PLA) (Cyllene: "Hmm. You look to be 15 or so. Quite old enough to work for your keep.")

But yeah, as much as I hate to admit this, Ethan, Kris, Leaf, Lucas, Dawn, Lyra, Nate, Rosa, Calem, Serena, Chase, Elaine, Victor, Gloria, Florian and Juliana never had their ages officially confirmed. At least similar to all of the above.