Saw a horn at an auction house a number of years ago that had a date of 1728 on it. Interesting horn in that the surface was more relief carved rather than the usual incised carving. It was a figural horn as well with animals and people depicted. My first impression was that it must be a recent piece, but the surface condition said otherwise and it is/was very likely correct. It no longer had an end plug in it, and the attribution was to Virginia. Other than this one, the oldest I have seen were 1740s period horns, which are few in number. Dated horns become more common in the 1750s on. Hope this helps some.
Dick