If we’re discussing where the cut-off for “high grade” comics would be, in terms of condition, then there’s an answer based on the eye appeal of a comic. It’s hard to see how a comic book rated at least Very Fine to Near Mint (VF/NM, or 9.0 on the 10 scale) wouldn’t be a “high grade” comic according to its appearance.
However, if the comic was produced last week and was a 9.0 condition, it would actually be “low grade” relative to the quantities which would be available at 9.4 (Near Mint) and above.
It would be hard to say that 9.6 or higher is required for a comic to be “high grade” (regardless of age) since there are older comics which may not even exist in grades above 8.0, particularly comics from the beginning of the superhero genre of the late 1930s to 1940s.
Using the decades of the comics as a guide, and offering possible cut-off points as a percentage of CGC graded copies (Universal and Signature), the following chart provides some extra information to the discussion.
There isn’t a definite conclusion to be made from the numbers, and there isn’t a definite answer to when comics no longer “look high grade” from an appearance standpoint (being in the eye of the beholder), but more than 3,000,000 CGC graded comics were analyzed to produce the chart above. That’s at least a little more information than we had before.