This usage is taken into account in the TLFi (chez, C) and it belongs both to the formal language and the everyday language.
C. [Avec des pluriels, des collectifs ou des singuliers de sens générique; le groupe prépositionnel désigne une classe d'êtres dont on décrit les traits ou les comportements spécifiques.]
There are several syntactic categories of noun phrases in which this preposition can be found. It can be translated in some cases approximately by "characteristic of" or "as pertains to", "for".
Il n'y a pas chez l'éléphant d'instinct qui, comme on le croyait, le mènerait vers un cimetière quand vient la mort.
(There does not exist an instinct characteristic of the elephant which, as we believed it, should lead the animal towards a graveyard when death approaches.)
Chez l'enfant en bas âge vous ne devriez pas utiliser ce médicament.
(As pertains to young children you shouldn't use this drug.)
1/ [plural nouns] chez les enfants, chez les physiciens, chez les artistes, chez les étudiants, chez les allemands, …
Do not confuse "chez les allemands" as meaning "amongst German people" and "as pertains to the German people".
Chez les allemands vous devez vous habituer à une sorte de politesse qui leur est propre. (Here, you can't say "l'allemand" (not correct), whereas in the next sentence you can do that without changing the meaning.)
Chez les allemands/l'allemand le sens de la discipline est plus fort.
(As pertains to the Germans their sense of discipline is stronger.)
2/ [noun modified by a quantifier] chez l'immense majorité des professeurs, chez la plupart des hommes agés, chez certains des sujets, chez presque tous les malades, chez quelques uns des étrangers en vacance ici, chez l'ensemble des vertébrés, …
3/ [term referring to a group including pronouns and periphrastic terms] chez la plupart, chez le reste, chez eux, chez ceux que nous allons étudier, chez la plus grande partie,
4/ [generic, singular noun] chez l'embryon, chez la plante jeune, chez l'insecte, chez l'homme, chez l'enfant en bas âge, chez le cep américain, chez l'allemand,
5/ [noun or pronoun referring to a given person] chez Pierre, chez lui, chez ce monsieur,
particularly used for writers, philosophers, thinkers chez Bergson, chez Sartre, chez Camus, chez Thomas Man,…
As the definition tells us this usage is specific to "beings" ("êtres") although there is much discussion as to whether things as embryos ("embryons"), vine stocks ("ceps") or plants are really beings. There is nevertheless a clear limit as indubitably inanimate entities can't be used in combination with "chez".
nouns that can't be used
- objects maison, chaise, métal, papier, livre (
chez le livre ancien, chez le livre moderne), tissu, vêtement, etc
- abstract nouns liberté, passion, feignantise, ardeur, fierté, abnégation, etc.
Addition due to a comment from user JD2000
I can't produce such a specific rule myself, and it is very specific if we can't link the species "book" to a genenral category. Nevertheless I am absolutely sure that I have never found such constructions for plain material objects or even not so plain, as for example preparations (chez la viande de bœuf, chez le fromage jeune, chez le travail de cet artiste, chez la peinture classique,…). Shouldn't we keep things well separated so as to preserve a clearly defined usage? —LPH
Is the "chez" in those examples the same as the "chez" in @Destal 's example though? I notice that Destal's example is in the plural, whereas yours are all in the singular. Could there be a difference in meaning analogous to chez les allemands = physically in their territory vs chez Balzac = in Balzac's writing? In other words would ce style particulier qu'on retrouve chez le livre ancien be more objectionable than cette odeur particulière qu'on retrouve chez les livres anciens? — JD2000
Yes, the meaning is the same.
No, there is not the least difference between "chez + sing;" and "chez + pl.". Whether you say "chez l'enfant en bas âge" or "chez les enfants en bas âge" the same meaning is conveyed; there is one difference in the frequency of use: the plural form is sometimes preferred; sometimes also this preferrence is such as to make the plural forms almost unique (for instance "chez le mammifère" is hardly used).
chez l'enfant/les enfants.
chez l'adulte/les adultes
chez les humains/l'humain
chez l'être humain/les êtres humains
chez les mammifères/chez le mammifère
chez les insectes/l'insecte
The plain reason for the singular in my examples is that the nouns are mass nouns; the concept of "chez", not meaning specifically "physically" as you say and as if by oppposition to "abstractedly" but meaning "amongst" has nothing to do in this question. Nevertheless, you raise a subtle question, which is "What is the real difference in meaning between the two?".
The question as regards user Destal's example (chez le livre ancien/chez les livres anciens) is whether there is or not an established usage for this type of thing. An ngram shows that neither form is used in the literature, which is a non negligeable indication; you can't find "chez le peintre moderne/chez les peintres modernes" either but I have no doubt (according to the principle of usage justified through analogous species) that you can say that; there is no question at all for the word "impressioniste" for instance. I believe that you don't find those forms user Destal refers to, and that you shouldn't go beyond established usage. A criterion for the deciding of whether the usage is possible, besides that of the objectionable one formulated in the definition of the TLFi that the noun could be that of a being (être) is that the noun must be that of something that is capable of a substantial enough evolution, but that is not good enough either: for instance, political movements and doctrines are just that, entities in constant evolution; yet you do not say "chez le MacCartheyism" and all you find is "dans le maccarthysme".
Note that in this example we are not at a loss for constructions to express the desired idea in a more versatile fashion than that allowed by the merely factual "chez": "odeur particulière qui émane des livres anciens", "odeur particulière [propre aux/caractéristique des] livres anciens", "odeur particulière qui presque immanquablement s'associe aux livres anciens", etc. I'll say one more time that there is no difference in the use of the singular and the plural, both are equally valid means of expressing the concept of "generic entity" and that's what concerns us when talking about this particular sort of noun used with "chez": they are to be used as terms naming a species. In the expressions I chose, "les" could have been replaced by "le" ("odeur particulière qui émane du livre ancien"). Both "chez les livres anciens" or "chez le livre ancien" are not idiomatic at all, I believe. Moreover, I see no definite argument for joining this type of word to the usage and therefore I think it best for the time being, without the support of further light on the subject, to keep to the norm that has left us the toil of ages past in the domain of coining usage.