Mark's age at end
In the episode THE VISION (S2,E26) the show starts with Mark and Lucas arguing about Mark's memories of his mother; Mark says that sometimes it doesn't feel like he ever had a mother. Lucas tells Mark that "You were 6; how can you forget?" Then in an episode near the end of the very last season, OLD MAN RUNNING, (S5,E24), Mark's grandfather appears in town. Lucas hates him and blames him for his (Lucas') wife's death. The old man says, "It was eleven years ago..." - that would make Mark 17 years old at the end of the series. I know the math is right, but he doesn't seem that old; 15 maybe?
Any thoughts?