My oldest is a ceramic Castle from my first trip to the Park back in 1967. It's about 7 inches tall, and 9 inches wide. Along with that I got a ceramic pencil sharpener with one of the Dwarfs next to a barrel. The shavings build up in the barrel as you sharpen the pencils. Cool.
I have numerous other items from the early 70's as well, including A-E ticket books, napkins, misc souveniers, the HM record and book.
My unofficial oldest item is a 1961 Park guide book. I say unofficial, because it was an eBay item. But I had to have it, since it was my birth year.