Even if he was apart of Akatsuki since then he would have been ten and that would have been about eleven years ago. While that is a possibility I highly doubt he was in Akatsuki since then but even if he was that would mean that he defeated Oro when he was that young. Not only that but during the flashback Itachi appeared as he is now he didn't seem to be much younger, he most certainly wasn't ten.

I believe he knew or found out about Akatsuki after he joined the anbu and then following Madara's instructions, they killed the clan and he joined Akatsuki soon after. I have no idea how long Akatsuki has been around though but it would appear that they have existed for quite some time especially if Madara is the one behind them.