Because he's Ferelden ambassador to Olrais , his duty demands him to be there.
Is his age referenced in one of the novels?
In the Calling, Eamon is 15, making him 45 during the Blight. Teagan is 7 years younger than his brother, making him approximately 38 when the Blight happens. If it's been 10+2 more years since then, then he's 50 in the events of the DLC. Older I could accept, white hair, more wrinkles. But completely altering his appearance is too much.
It's like they took this beautiful CC engine and just didn't care how he turned out, pressing buttons at random to create him.
I refuse to believe it is him till I hear differently in the game. 
And if he IS the antagonist and we have to kill him, that will kill the franchise for me. One reason I never got the DSC dlc because 1) it was crap to begin with and 2) at one point you kill Teagan.