There is no specific age of accountability, for it is far too complex a function for it to be. I'm sure we could draw up a "common" age of accountability, but to obtain an answer to this question would require an analysis of a person in particular.
It depends entirely upon the knowledge and intelligence of a person and this is a factor that is different for each and everyone human on this planet. If an individual understands the difference between right and wrong seems to be a good starting point.
But then, there is the fact that the young, even though they know the difference between right and wrong, are often quick to judge and quick to seek action. They are emotional and not always logical.
Thus it is hard to define an acutal age. But to be frank, I think once an individual learns of right and wrong and understands the respect that they should give to their fellow man, should they be considered accountable. If someone doesn't respect life or liberty or the pursuit of happiness, then we can't trust them to be a functional member of society. So this draws up a new question I believe: How do we deem someone as a disfunctional member of society - or put more simply - how do we define someone as "dangerous" to society? And what should we do with them?
I suppose I haven't answered your question, but drawn out it's complications. Like I said, I personally believe that we should judge people on their understanding, their intelligence, their comprehension. And to judge that adequately an individual must be studied.
Perhaps an age should be set, like 18 and if an individual isn't responsible enough to be accountable then they will be punished. Although, by the time we would know they weren't responsible, it would be too late and now we are perhaps getting into a situation similiar to one in that movie with Tom Cruise, about the future. I can't remember it. Minority Report. That's it.
Well, I hope my answer helps at least a little.