It is easy to get into the mentality that when you are in the early levels of Japanese you should choose books based on your level. If you are a beginner, start reading books for babies. As your level rises, work your way up to reading books for children, junior high school students, high school students, and then whatever you want.
This is a bad idea, unless you really find pleasure in reading books made for children. There is a reason why adults don’t read these books. They are boring and don’t appeal to the adult mind. The only time you should ever read a baby book is if you have a baby.
Don’t read what you wouldn’t in your native language.
I used to often make the mistake of not following this, and ended up frequently disposing of a lot of Japanese material. By now you know what you like to read. Why should you throw away this knowledge you’ve attained just because you are learning Japanese.
An additional problem is that with baby, children, and young adult books, there is often very little kanji. You may be thinking that this makes things less painful, but you will eventually come to realize that kanji makes reading Japanese easier. Assuming you have went through all the kanji using Anki, reading in only hiragana and katakana will be a huge hindrance to you.
This doesn’t mean you shouldn’t adjust your reading material to your level. You definitely should. However, you should adjust it based on difficulty level, not on age level. Different genres for adults have different difficulty levels. If you are only around level 20, don’t read a book on politics, read a book that involves a time traveling high school girl. If you are level 30, don’t read a book on the inner workings of the Edo era during the 1800s, read a mystery book about 3 people hired to trap a man in an elevator in an attempt to mend his cheating ways.
I also recommend books that are written at a young adult level but have large appeal to adults. This would include Harry Potter or books by Roald Dahl.
Reading in Japanese should be pleasurable. Base it on your interests, not your level.