TITLE: When to learn category theory?
QUESTION [123 upvotes]: I'm a undergraduate who wishes to learn category theory but I only have basic knowledge of linear algebra and set theory, I've also had a short course on number theory which used some basic concepts about groups and modular arithmetic. Is it too early to start learning category theory? should I wait to take a course on abstract algebra?
Is it very important to use category theory facts in a first course in group theory, ring theory, fields and Galois theory, modules and tensor products (each of those is a one semester course), would that make it a 'better' course?
I was unsure to learn category theory early but this post Mathematical subjects you wish you learned earlier inspired me to ask you given my background.

REPLY [19 votes]: I've already answered a similar question on MathOverflow here.
Here are some thoughts of mine:

category theory (seen as a language) should not be teached just in advanced courses, but it should be developed into the basics courses, in a very gradual way;
some elementary concepts are so simple that also a first year student can understand them, if these concepts are presented in the right way:
  for instance you can see a category just as a graph with operations and functors as graph morphisms preserving the operations (this definition is not more complex or abstract then group-group homomorphism and vectorial space-linear map definitions; because these concepts are so simple way not introducing them early?
learning category theory helps in making connections between many different concepts, because it shows the deep unity in maths; 
category theory is first of all a language, and so it gives us a new way of reasoning; this new way of reasoning requires some time to be fully assimilated, and this assimilation could require years; for this reason I think it's best starting to learn category soon; 
having category theory help to learn new maths: for instance I've learned category theory for my interest in logic and foundations, then knowing those concepts helped to understand constructions in algebraic topology and algebraic geometry faster then what I would have done without it;

Other things can be found in the link above.