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Burns rewrote the song based on 'the old song' so it must be at least 18thc. There are 17th century ballads on the same theme which no doubt inspired the Child Ballad. If pressed I'd say it's a typical stall copy rewrite of one of these, but I haven't seen a stall copy earlier than 1800, Pitts being the earliest I've got, although it was widely printed in the 19thc.