To a point it really is pointless trying to ascertain what Ash's age is - the only real indication we have are vague animé references that state a year has gone by, or whathaveyou. If a proper age must be decided on, we'd have to bear in mind that puberty tends to strike two or three years after the age Ash was when he first left home - which raises a series of questions about his, his mother's, and the writer's ability to think certain things through, but that's not what we're talking about here.
Now, has Ash hit puberty? I'm thinking not, from a realism-standpoint; sure, his voice has apparently, all-of-a-sudden, changed - but from what I've heard of it, it isn't any more deep than the last one. It's just different, much like when someone goes home and dyes their hair a completely different colour - it isn't any longer, no time has passed in which it might have grown or changed naturally; it's just a different colour. And yes, people will notice, so there'd be no use pretending nothing was different.
Of course, nothing would be said of the real signs of puberty,* but even if we couldn't expect this, we should at least expect some change in his level of cognitive development - yet he reasons and argues as if his mind froze at age ten. Possibly it did - I don't know, I can't read it or anything. Even if he'd only aged two years, though, his maturity level would have advanced - so unless in Ash's case there's such a vast difference between ageing and maturing no possible connection may be drawn between the two, I wouldn't be so fast to state that Ash was anywhere near turning twelve.
*Body hair, smells, emissions. I myself went through puberty around ten years ago and since then have had to wear deodorant, change my clothes daily, wash just as often - and still my t-shirts have this rather strong smell of hormones. This may in part be due to the fact that I like tight t-shirts, so the underarm area of them is in close contact with my own; however, I'm sure the days upon literal days Ash and company spend out in the wilderness, in the same items of clothing, without showering, would result in something of a similar sort. Sure, they could "wash" their clothes - basically meaning they could splash water on the dirty bits and hope for the best - but I have to wash my t-shirts daily. And the smell remains.