Well, Actually about 25% in Johto, a year had passed. In the Pikachu short before movie 3, they were celebrating the year anniversary of Ash meeting Pikachu, which was on his 10th birthday. So this is after going through one full region, a small sidequest(Orange Archipelago) and a quarter of Johto. So now say the rest of Johto, Whirl Island, and a small part of Hoenn counts for another year. Use the rest of Hoenn, Battle Frontier, and a bit of Sinnoh, and another year has past.
So basically, Ash COULD be 13, turning 14 a bit in Ishuu