Why can't a count noun denote a class when used alone without any determiner?

Solution 1:

There is a constraint on the well-formedness of English noun phrases that an NP with a singular count noun as Head (of the nominal), must have an overt Determiner. Consider:

• What's this?
• *Pen (ungrammatical)

And then compare this with:

• What's this?
• A/my/Bob's/some/the pen. (grammatical!)

So, in short, it is not that in the string Boy is young male human, the word boy cannot represent the class of boys. Rather, it's the case that the string is not well formed and is therefore uninterpretable.

Of course, nouns occur mainly in noun phrases. Noun phrases are complex, and like other types of phrases can often be thought of as constructions. It is only once a noun enters into a construction in some way that we can give it a salient interpretation.

Notice that when CamGEL refer to Boy in that quote they are referring to the lexeme boy not the singular uninflected form of the noun in particular.