should there be a definite article before "eye level"? (a quote from Salinger)

From Salinger's Zooey:

There were scars much nearer to eye level, too, from a rather awesome variety of airborne objects—beanbags, baseballs, marbles, skate keys, soap erasers, and even, on one well-marked occasion in the early nineteen-thirties, a flying headless porcelain doll.

I'm curious why there's no definite article before "eye level". Is level treated as a mass noun here? Could we add the before "eye level" without affecting the meaning?

Answer

As Jim states in a comment, "eye level" is a rough approximation rather than a specific measurement, in the same way that "room temperature", "ice cold", or "knee high" are. (And yes, I'm aware that the first two can have defined specifics, but in general use they are approximate.)

Wikipedia has a listing of the average height for a number of countries. A quick approximation from this list puts average human height (very) roughly around 155-158 cm. Thus, you could rewrite the start of the sentence like this:

There were scars closer to 157 cm high as well...

You certainly wouldn't add a definite article before "157 cm", would you? So the answer to your immediate question is "no, do not use a definite article in that context."

You could, however, write a sentence like this:

The insect flew past, inches from his face, and precisely at the level of his eyes.

...precisely at his eye level.

Note that this moves the context from a generic approximation of human height to a specific instance of one person's height. Instead of being an approximate placement of objects in an undefined area, it's a highly specific location relative to the person in question. (Even if we don't know how far off the ground his eyes are located, we know they must be some distance away from the ground.)

Your broader remark about level as a mass noun is a reasonable attempt to intepret the word, but incorrect. In standard English, there is no common usage that approximates to "[number] [units] of level". It is always used as a count noun:

The house has three levels plus a garage.

That is wrong on so many levels.

I completed the last levels of the game!

There may be some technical field that uses level as a mass noun, but again, it is not standard English usage.