my arguments to support this theory are as follows:
1) He's the most mysterious character we've seen in the game so far!
2) He is obviously much older than he looks, since according to what we see, he is at least as old as Garth!
3) When we cleaned up Fort Marrok, River confirmed that it was Jarl Gunvaldsen's medal simply because of the smell. HOW THE FUCK COULD HE KNOW THE SMELL OF SOMEONE WHO DIED MORE THAN 200 YEARS OLD? even though River was older than Garth, Jarl was just a ghost, and his body has already turned to dust for centuries!
4) When we ask him to change our names or titles, he does it instantly! Even if he were the best bard in all of Savarra, and had more informants in the Frost Marches than Varys and Littlefinger combined, he wouldn't have done it that fast!
5) When he was introduced, it was said that he could change his name by distorting time and space, changing everyone's memories!
6) While his avatar is a catboy, dark-haired and wielding a golden citole, being a "god", he can take on the shape he prefers, so it would make sense for him to let his hair turn white, thus, even though he keeps the beauty appearance, no one would suspect your age! I mean, look at this!
Do you seriously think that someone in a practically medieval world would be able to keep that perfect skin, close to 40, in a place like the Frost Marches? Even more considering that he is not an elf, kitsune or vampire!