Think you've missed the point I was trying to make.
Yes, with demographic trends you can predict births and deaths, what you cannot predict is how people will vote at a given point, mainly because of the unpredictability of political events. You alluded to the fact that so called 'gammon faces' who voted remain, might change to leave voters and swing it for Brexit... but not perhaps if it leads to the downfall of the Tory party?
Even before a second referendum can be considered, other events have to play out. EG. Will Labour MPs who came from leave areas cross the floor and vote for May's deal? Will Tory MPs from leave areas vote for measures to defeat Brexit? Perhaps the only thing for sure, is the DUP will not vote for anything with unlimited 'backstop' in it? Any or all of these events should they take place could affect the vote in a second referendum, just as much as if 'x' number of old people die and 'y' number of new voters come on to the register.