Since Brexit didn't happen when promised, it's only reasonable that the original referendum is also extended to include people who have since reached legal voting age. Anyone born on or after June 24th, 1998, should have their votes added to the existing results of the original Brexit referendum.
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This isn't a second referendum. It's an extension of the original referendum - just as Brexit has been extended (and the referendum is part of Brexit and therefore due the same extension) - for those who have reached legal voting age in the meantime.