12 year old veneers lightening near gum line of canine tooth. What can this be? (photo)

Thanks for the photo, it is helpful. It is possible there is breakdown in the veneer bonding attributing to the color change. Or maybe be time for a new veneer. This is my personal opinion and not a diagnosis.

There seems no visible lightening in the picture, but I recommend you to see a dentist if it bothers you. It needs to be checked if there is a gap between the shaved tooth and the crown by having an x-ray if possible.