Given that the Wheel is cyclical, there can't really be a 'first' Age, that would imply a beginning. That, I reckon, is why Robert Jordan always wrote 'called the Third Age by some', it's not a real designation, it's just people calling it that.

As to why people would call it the third, well, obviously it can't be the first because there was the Age of Legends just prior. And if the Age of Legends was the culmination, the perfect absolute of what could be reached (in Third Agers' opinion... people always seem to think that the past was better than the present, there are Golden Ages in most every culture, and even today you can hear people talking about how things were so much better 'back in the day'... only in fantasy does that ever seem to be really true though), then there must have been an Age before that, the age of the beginning. All it would take would be some widespread loss of knowledge (like another Breaking of the World, or, heck, a worldwide Dark Ages) and people would probably start calling their current Age 'the Third Age' again.

Anyway, as to what brings us back to our current Age (The 'First' there's really no way to know. I mean, we know the history of our world fairly well back to the solar system's creation, so it's hard to see where previous Earthbound Ages occur, either RJ was using certain liberties with our timeline, or else the last Age (the seventh, according to the seven Age hypothesis) will end with a Big Bang...