Vincent was 26 when he was put away after uncovering Hojo's affair with Lucrecia (who was never married to anyone, by the way). Thirty years went by before Cloud woke Vincent up, but since his body didn't age while he was confined, his age is still listed as 26 even though it's technically more like 56. Sephiroth must be about 30, since it was thirty years ago that Hojo and Lucrecia had the affair that got Vincent put away.