Continuity mistake: (Spoiler Alert) the day after Hancock finds out that Mary also has powers, he stabs her with a large fork with two pointy ends. As expected, they do not puncture her skin. The pointy ends bend toward each other, making a triangle shape. But in the next shot, when Mary snatches it from his hand, the tips are no longer in a triangle shape, just bent the wrong way a little. Mary snatches it away by the handle, so we know that she could not have taken it by the points and bent it.
Continuity mistake: When Hancock puts Mary on the hospital bed after she's been shot, the blood on her shirt changes drastically from shot to shot.
Continuity mistake: In the end, when Aaron and his parents are walking at the fun fair, they are holding an ice cream. The brown Italian ice cream of the boy changes from right hand to left hand between shots.
Continuity mistake: As Hancock enters Ray's house through the hole in the wall, you can see Hancock is not wearing anything around his neck. As this scene progresses, his gold eagle chain is visible at times.
Answer: That was never answered in the movie, though she did say they were at least 3,000 years old.
pross79