My guess, after a short check via the google N-gram thing, is that the use of the two terms as opposed to each other started around 1978, in Dark Imaginings (http://books.google.com.vn/books?id=0zNaAAAAYAAJ&q=%22low+fantasy%22&dq=%22low+fantasy%22&hl=en&sa=X&ei=XttTU9KCIcb1kQXJq4GQAQ&redir_esc=y), a work of literary criticism by Robert H. Boyer, Kenneth J. Zahorski (which I haven't read). The term "high fantasy" seems to have been around since the mid-nineteenth century, though.
Editing to add: The term "low fantasy" was around earlier in the 1970s, but it looks like it was then being used by psychologists to describe children who didn't fantasise much, as e.g. "Seven- to nine-year-old boys were initially administered part of the Michigan Picture Test and then categorized as High and Low Fantasy on the basis of their scores..."
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The term "high fantasy" seems to have been around since the mid-nineteenth century, though.
Editing to add: The term "low fantasy" was around earlier in the 1970s, but it looks like it was then being used by psychologists to describe children who didn't fantasise much, as e.g. "Seven- to nine-year-old boys were initially administered part of the Michigan Picture Test and then categorized as High and Low Fantasy on the basis of their scores..."