Well... Well... Well...
This is a topic of dissision, rather than a topic of standard things...
Things we know:
1. Satoshi started off being 10 years old. [Episode 001]
2. On the episode of the Earth Badge in Viridian, in the Kantou League, Ash turned 11. [Episode 063]
3. On the Pichu Bros. special, it was mentioned that, that day was the anniversary of Satoshi and Pikachu. That means 12. [Pichu's Special - More or less in some part above Episode 140, taking into account Charizard's going]
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Plot One:
That way Satoshi is "aging up" every about 65 episodes. The series have 468 episodes untill Battle Frontier. That means that Satoshi would be about 17 in logical terms, but let's say he's 16 after going slowly etc...
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On the other hand...
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Plot Two
Taking into account the fact that Kantou Series last a year [EXACTLY], and since until the end of the League it is 1 year and 1/4 (since the League lasts 1/4 of the episodes), every main saga lasts 1 year, plus the league which lasts 1/4 of a year. So, Satoshi being 11 and 1/4, goes to the Orange league. Orange League lasted only 24 episodes, I guess that: Satoshi going to take the badges took about 2/8 of a year, while beating the League would take 1/8 of a year. That means that Satoshi goes back to Kantou being 11 and 5/8. I guess that near the anniversary of the Pichu Bros. he turns to 12. And since we know that Satoshi started the Kantou league in the age of about 10, and it ended when we was 11, that means that a league lasts 1 year. So, by the end of the Adventures in Jouto he is 13. On the League he goes 13 and 1/4. After that we goes to Houen. It takes one year to beat the Gym Leaders (so 14 and 1/4) and after the league ends he is 14 and half. As for the Battle Frontier, I suppose it lasts the same as taking the badges. So, after the Battle Frontier he is about 15 and a half.
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I'm mostly for the second plot, as it seems more logical in my opinion.