I usually go by the common sense and ten apply math to them. Assuming in the anime, you have to be 10 years old to start your pokemon adventure, we can say that Gen 1's Main Characters are 10 years old.
Then Gen 2, is 2 years after the events of Gen 1, that's basically makes Gen 1 MC 12 years old and the MC of Gen 2 10 years old.
After, Gen 3 has no background of time it has been, we can assume that it started around the same time as Gen 2, making both MC in that game 10 years old as well.
Same goes for Gen 4, they, too, should be around 10 years old.
In gen 5, they are supposedly to be older than the other Gens, so we can guess it's in their teen years, presumably 14. While the second set of MCs should be around the same age, making the first Gen 5 characters 1X, X = the years that it had gone by.
In Gen 6, the three kids and both MC are starting their adventure so we can say they are also 10 years old.
Then we calculate it by adding all the games and stuff together, I think the age of the MCs across all Gens should be like:
Gen 1: 14 Gen 2: 10 Gen 3: 10 Gen 4: 10 Gen 5a: 14 Gen 5b: 10 Gen 6: 10
Nowhere in the games does it say 10 years old is the official trainer age. That's the anime, which is a whole different continuity.
The two confirmed ones are Red, who is 11 accdg to a manual, and Hilbert/Hilda, who are 16, tho i don't remember where i read that...
It kinda makes sense - the Japanese have a whole different take on age of maturity compared to Americans, so the descrepancy makes sense. Going from that, i daresay Ethan and Kris/Lyra, Brendan and May, and Lucas and Dawn are all between 11-13, Rosa and Nate are 15-16, and Serena and Calum are 16.
Now playing: Nocturne, Digital Devil Saga duology, Strange Journey, DeSu: OC, DeSu 2. Yeah, i'm on an SMT high.