Okay this is not really a fanfic but this is an argument on how I think that Ash is not still 10 years old.
Argument one: Serena
Okay, so I know that you are probably confused about how can Serena factor into this. But bear with me. Serena, if you were to look at her first outfit and her other outfits, it is obvious that Serena has, well, breasts. And I don't mean just starting to get some like she has A cups, she has B cups, like B5 or there is the episode where Serena was wearing that Fennekin costume for her Pokevision video, and she has curves, where she has a small waist and kinda hips, instead of having her figure be straight down if she was 10. So that would mean Serena is likely 13 or 14 years old.
It is also kind of obvious that she has a crush on Ash. And what kind of 13-14 year old would have a crush on a 10 year old? Plus even if Serena was 10, it is kinda, in a way, obvious that AmourShipping (Ash x Serena) might become canon. And that means that they will probably date or kiss in order for that to happen. And don't you think 10 is a bit too young?
Argument two: Ash's size and voice.
Ash's voice kinda changes from a high kinda pitchy voice to a little bit deeper voice. I know that this is probably due to the voice actors for Ash had changed, but maybe they are trying to tell us something. Then there is Ash's size. In the first few seasons Ash had to sometimes tilt his head up at people who were obviously older than him, but in the recent ones Ash is almost about the same height as people who are probably 16 and up, and only have to slightly turn his head to look at them. Then there is the XY episode where the group meets Sawyer, who had just started his journey. And the legal age in Pokemon to become a trainer is 10 years old, and if you were to put Ash and Sawyer side by side Ash is visibly much taller than Sawyer.
Argument three: Days passing.
Okay so there has been obviously more than 365 episodes of Pokemon. I mean it started in 1999. So anyway, in some of the episodes there is seen where more than one day passing in a single episode. Like maybe instead of one day to have their adventure or whatever, it takes them two days. And there are even a few episodes were a couple of days have passed, like in the episode 'A true Blue Swablu' Brock, Ash, and Max decide to stay a few days so May could take care of a injured Swablu to full health. Then in the movies with Ash in them it is quite obvious that two or three days have passed inside of the movie.
So there are 887 episodes of pokemon in all and if you were to take the episodes where two days passed in each one, I guess you would have around 815 episodes and the ones where only one day passed, 72 episodes. And if you were to get 815 and multiply that by two. You would get 1630, thats is as many days that Ash had been through since the start of his journey. You add the single ones, 72, with that number and you get 1702. There are 365 days in a year so divide 1702 by 365 you would get 4.663... Basically estimate that, and you would see that 4 or 5 years had passed by since the start of his journey. So if Ash was 10 when the Pokemon series started he is now 14 or 15 years old by now.