In LOC
A New Arrival

Rand studied Taim as the man and his escort crossed the courtyard. At least fifteen years older than himself; thirty-five, then, or a few years more at most. ... Rand had clung to sanity for close to two years, so far. Yet in front of him he had a man who must have managed it for ten or fifteen. That alone was worth something.

Clearly Rand thinks Taim is in his mid-late 30's and must have been channeling for 10-15 years.

However, later RJ stated the Taim was significantly younger

RJ's blog 4 October 2005 "ONE MORE TIME"
Rand misjudges Taim’s age because when they meet, you might say Taim has been rode hard and put away wet. He has just finished a long and difficult flight to reach Caemlyn, the one place where he might find refuge instead of being hunted — along with other reasons — and that has a wearing effect on anyone. Now that he has recovered, he doesn’t look so old.

This is also suggestive that he is NOT a Forsaken (why would one of the Forsaken look haggard?). But could well have been intercepted and put through the meat grinder so to speak ("along with other reasons" ).

Sanderson was cagey in what he had to say about Taim being turned by a 13x13 circle:

The Gathering Storm Book Tour, Books and Co., Dayton, OH 11 November 2009 - Tim Kington reporting
Q: Was Taim turned by a circle of thirteen?
A: RAFO. I will say that he was never a very nice guy.

Clearly Taim was never a nice guy, but that doesn't automatically mean he was already a DF (look at Elaida) or that he wasn't turned. Now that we're seeing others get turned in the BT it seems more of a possibility to me that Taim could have been turned... hell they may have done it to him even if he was already a DF ... just to make sure a Channeler of his immense strength stayed on their side.