What is the threshold between high Aspect Ratios and low Aspect Ratios?
The boundary depends on the type of aircraft. Fighter wings will be called high aspect ratio when the same aspect ratio in a glider wing will still be called low.

Generally, values below 5 are low, regardless of aircraft type.

With passenger aircraft, the boundary might be around 8.

Gliders will only have a high aspect ratio if it surpasses 20.

Still, there is no defined boundary and in the end the choice depends on the whims of the author.