By ALISSA WILKINSON At any given time, there’s one human on our planet who’s the oldest one alive. Most of us will never hold that title. But, as director Sam Green reminds us in his charming and often moving documentary, “The Oldest Person in the World” (in theaters), we were all once the youngest person on Earth, if only for a fraction of a second. Time moves inexorably, and the real subject of Green’s film is how we deal with the implication that someday, all of us who are here will be…
