Vertical forehead dent. Is there anything I can do to fix this? (Photo)

You have two options to treat it, fat injections and a thin bone implant. The interesting question is whether this is due to soft tissue atrophy/thinning or a bone defect underneath. While it is most likely the former such a forehead development is not normal or part of any natural aging process. If it was not there at all when you were younger and has only appeared as you have gotten older, this makes me think of linear scleroderma...which is exactly how this appears although it being in the dead center of the forehead is unusual as its always follows the path of the more lateral supraorbital nerves.

Your photographs demonstrate a shadow which is visible due to contour irregularity most likely at the level of the frontal bone. Although bone contours can be addressed with bone remodeling, this tends to be a bigger operation. A simple solution would be to use injectable fillers to reduce the depression which in turn would reduce or eliminate the shadow visible in your photographs. This is a simple office based procedure with no downtime other than mild redness and swelling in the treatment are for a few days. This would likely last a year or longer and could be repeated at your convenience. Consulting with a surgeon experienced in fillers would be a great place to learn more about this option.