There is a woodcut from 1692, England, depicting a little devil with a scythe...making the grass lay down in a circle!! So...they have been around forever, I suspect!
Apparently, the farmers in England who grew up as children in the 20's and 30's were totally used to crop circles in their fields and would play in them. They were just a natural part of their world.
Check out the database. The earliest recorded ones in the database are from the 1950's.
Click on the url below and scroll halfway down the page. On the left are some blue links titled: Quick Archive.
Click on 1950's or 1960's or 1970's. In the 60's crop circles were big in Australia and France.
Paul Vigay's site is an excellent one - with great articles too.