Fred, I don't know why - age, presumably - I constantly forget to check the online version of the OED, but I just did that. This very year they added an example from the Chicago Sun Tribune in 1949: "The jury is still out in the case of TV against athletics."
As far as the UK is concerned, The Times had a reference in 1984 to a company's potential to become a high-growth business, a matter upon which "the jury was still out."
So, it would seem that it is at least 60 years old.