There's not a specific age where this work has to happen. Reading a variety of challenging books aloud to the class is one of the best things schoolteachers (especially early grades) can do for their students. Listening/language comprehension is incredibly important and underemphasized throughout most schooling.

Reading per se (i.e. decoding written symbols into sounds/words, at least in English or similar languages) is a quite discrete skill that takes something like 6–12 months to learn to basic proficiency, working an average of, say, 15 minutes per day with direct guidance. It has some basic pre-requisites (attention span, interest, recognizing the alphabet), but can be done at any age; some kids might be willing to learn to read at age 3 or 4, but it can certainly be started at age 8, 12, 20, or 45.

After that, speed and fluency improves with additional practice. Like anything, it's easier to get very fluent if people start younger because they have fewer other obligations. Someone who learned to read at age 4 and then spent hours reading for fun every day for 10 years is going to be far ahead of someone who learned a bit starting at age 8 but never had much help, and afterward only occasionally skimmed some magazines for the next 5 or 6 years but mostly spent their time on something else.