Site contributor Lippke spotted an irregularity between the manual and the game's plot. The manual states the age of Vincent Valentine as 27. However, remember how Vincent had previously worked for the Turks before Sephiroth was born?
We can safely assume Sephiroth must be at least 21 years old. If Sephiroth was born twenty one years ago... then Vincent must have been six years old during that time, and hence way too young to be a Turk. So, how is this possible?
The human mother of Sephiroth was a scientist named Lucrecia - Vincent's love. She was seduced by Professor Hojo and impregnated with Sephiroth. Vincent vehemently opposed this plan, but was sadly 'disposed' of by Professor Hojo. The professor 'scientificically altered' the ex-Turk and 'put him to sleep'. It is possible this form of 'sleep' is a form of suspended animation. As Vincent 'slept' in his coffin, the years would pass by until that fateful day Cloud intruded into the Shinra Mansion. When Vincent at last re-emerged from his coffin, he was still the same age as when he was first entered.
Thanks must go out to R. Dondradill for making some corrections.
Also, The 13th Dark Knight has his own take on Vincent's age here, which may help solve this puzzle. If you end up confused, don't worry, you're not the only one! We can safely conclude Vincent is one of the most mysterious characters in the game, perhaps more so by accident than design.
UPDATE (March 2006): Several people have noted (and I myself have noticed) that in official materials for Final Fantasy VII: Advent Children, Vincent's age is listed as being 27. Considering that AC takes place two years after FFVII, the fact that Vincent's given age hasn't changed during that time is a very strong indicator that he hasn't aged since Hojo experimented on him, and even after his re-awakening.
UPDATE (July 2007): The issue with Vincent's age gets even more complicated. In the Japanese and US official game guides for Dirge of Cerberus, Vincent's physical age is given as 30 instead of the expected (in light of AC) 27. Thus, this bit of information makes the bold claim that Vincent has aged between his awakening in the original FFVII and DC, which is set three years later. Is this a grievious error on Square Enix's part, or is it AC's age information which is incorrect?