>>54563082They're older than younger kids but younger than older teens and adults. The artstyle makes them look more on the younger side, but I think a roughly 12-14 age range is a safe assumption.
One interesting point in the timeline is that according to the History class, the Professor's research which led to the invention of the Tera Orbs happened only "approximately 10 years ago." Corroborating this with the Area Zero journals, it is established that the Lighthouse Laboratory and the eventual invention of the Time Machined happened a considerable amount of time after this breakthrough, and Arven himself was not born until after Koraidon/Miraidon was brought through the Time Machine. This would imply that Arven is less than 10 years old.