You mean reading in general or reading specific things like articles, books, manga?
For each of these it will vary. The NHK Web Easy articles are definitely a good place to start if your grammar is around N4 level or above.
Reading manga can be deceptive, because there is a ton of different genres, themes, characters speaking in different styles and there is a prevalence of short colloquial expressions and blurring the lines between politeness levels.
Books and stories can also be a mixed bag. Like the previous story from 大川先生 is actually a decent one, because it has a moderate amount of kanji and only slightly archaic forms. The current one seems to be on the tougher side. But books aimed at children or young teenagers (middle schoolers) are probably a reasonable general direction.