Watch the movie until you see the chief of police argue with Lauren Randall's father. Lauren's father states that his daughter was 18 when she died, but at the end of the movie, if you look at Lauren's tombstone it says (1979 - 2000). If this was the case, she would have been 21 when she died, so either the undertaker got the date wrong, or Lauren's father doesn't know how old his daughter was when she died.