As for his age, on most things I have seen it is stated unknown or as age fifteen in the television series and seventeen in the OVA. Few have claimed him to be a year older than the rest of the Gundam Pilots though. I believe it is actually unknown, but from assumption, all of the other pilots are of the age fifteen.
It is also shown in a scene from "Episode 20: Lunar Base Infiltration" from the anime where Colonel Une is overlooking his file information when he is trying to infiltrate OZ. A clear capture of this scene is found in The Art of Gundam Wing book. On to file it clearly says his weight. Along with it, though the labels are cut off it has the number 15 on the first line and 160 on the second. One can easily assume that the fifteen is for his age, while the one-sixty is his height. Again, that could be circumstantial. He could have simply stated that as his age by example of his fellow pilots' ages.
On Japan website Mobile Suit Gundam Wing Official Web his age is considered unknown but he is presumed to be fifteen in the television series and thus presumed to be sixteen during Endless Waltz movie/OVAs.
Again, through Episode Zero, it is determined that if he is Triton Bloom he was born in AC 180, at age two he was separated from his family due to their wagon being caught in crossfire between Alliance forces and rebels in AC 182, and was picked up by Mercenaries when he was four.
Episode Zero gives no true indication to how old he is during year AC 190 and but notes he left to go to space when he was fourteen after his mercenary squad was destroyed. April 7 195 is when the five pilots were sent to Earth. Again he could be fifteen or he could be fourteen should he indeed be Triton Bloom. It is never stated what month he was born.
At the end of his segment three articles are given. The introduction says,
"Trowa's Story: Trowa mysteriously introduced himself with the words, I'll call myself Trowa Barton. The shocking 16 years of his life are now made clear!"
Since Episode Zero was written after the OVA series Endless Waltz, it can be assumed he was indeed the same age as the other boys. Still, there are different views as to whether Episode Zero can be considered official.
As for those who report him to be a year older than the other boys, I consider that fine since no one knows. However, some say he is older simply because he is taller. That is a poorly executed excuse. The five Gundam Pilots are all mid-teens and will have growth spurts. They are all different nationalities and have heritages that can effect how tall they are and will be. Height has nothing to do with age.
It seems some believe for fact that his zodiac sign to be Aries due to an unofficial Japanese art book. Whether that is true or not, I would not know. A few other places noted predictions of him being a Capricorn and on NationMaster.com his birth date was said to be July 22, AC 178 which would be the cusp between a Cancer and a Leo.
Other than that, nothing else has come up. In view of what has been found, he ultimately leans towards being fifteen during the television series over anything else.