It all seems abit perverted, but hey it's Japan. No racism intended. Ash should be about 14/15. For example, when they got to Viridian city in Kanto. Ash went, "I can't believe a year has passed". Well he's definatly over 11. You can see Ash looks more matured, with the clothing and facial expressions.
I knew a year had to have past!
Let's just say it took him a year for each region (excluding the OI)
Now, I don't think the The Orange Islands took him a whole year. it was probably no more than half that.
If I recall correctly there was a few months break after Kanto, right? add that in with however long the OI took, and I think it's about a year between the end of Kanto and Johto's Beginning.
so:
1 for Kanto, so he's 11 at the end.
I'd say about a year has past by the time Johto starts, so he's about 12.
13 by the time it ends and Hoenn begins.
I'm not quite sure how long they spent in Kanto the second time (Battle Frontier)... But I'm sure it wasn't a whole year.
I'd say he's about 14/15 right about now.
Same for Misty.
May is about 11 or 12 right now. (though, I don't recall if they ever stated May's age)
Dawn, it was clearly stated that she's 10.
Not quite sure about Max and Brock... I think Max is like 9 or something. And Brock is maybe... 18? (did they ever state Max or Brock's ages?)