None of this is really known.
He doesn't age in the anime, as you know. His birthday has only been mentioned since the beginning of the anime, and later the inaugural of the BW anime. Both of which he was 10 years old, so he doesn't age.
This was intentional, I'm pretty sure. Game Freak just want Ash to stay his young, accident-prone self. It wouldn't be very entertaining watching a 25 year old Ash walk around, hanging with people younger than himself, and possibly even getting sick of journeying around for years on end.
Also, it would be a huge disadvantage for the younger generation. The younger generation of Pokemon would still want a chance to watch the entertaining Ash, rather than him years later.
So none of this needs to be known, really. It's like The Simpsons (in particular) - Maggie, Lisa and Bart should be well in their 20s now, and for Bart his early 30s. It would just be sad to watch them grow up and get... 'mature'. o_O
So yeah, he's 10. Still... Since he hasn't age since the beginning of the anime, and BW001, his birthday is also unknown. Sorry.
Hope I helped. :)