IceBeing--I live in Japan and read and write Japanese fluently.
Japanese is not written with an "alphabet." It is written with two (very easy-to-learn) syllabary systems (hiragana and katakana), plus Chinese characters (kanji).
Although theoretically, elementary schools "teach reading and writing from zero, starting with hiragana and katakana" in practice the large majority of kids can already read both (certain hiragana, and usually katakana as well) by the time they enter elementary school at age six, and are generally pretty good at writing them too.
These syllabary systems are so easy to learn that it's super easy for parents and other unqualified people (daycare staff etc.) to teach them, and for kids to pick them up little by little by looking at picture books--rather like Finland's writing system. I would be very surprised if I met a five-year-old who wasn't at least reading hiragana.
The schools "run over" the syllabary systems very quickly over the course of a couple of months in the first grade, then rapidly launch into kanji.
By the way, the difficulty of reading kanji does not stop kids from reading children's books, including their textbooks. That's because of the practice of using "furigana" in children's books.
en.wikipedia.org/wiki/Furigana
Kids' books use kanji just like adult books, but they insert the hiragana (which shows the pronunciation) in small font above the kanji, so that kids can gradually pick up more and more kanji, while at the same time being able to actually read the book. The difficulty of reading kanji therefore does not pose a barrier against children doing things like reading books and learning to love and enjoy books, acquiring general knowledge, enriching their vocabulary and being able to read their school textbooks.
By contrast, in the case of English we have no furigana system, which means that children cannot read English books until they have actually mastered our difficult orthography system.
My own observations of the differences of writing systems here is actually yet another reason why I have decided to start my daughter to read English early. My observations of Japanese kids quickly being able to read children's books in their own language, makes me feel even more strongly that it is a good thing to be able to read sooner rather than later.
Plus, I've heard too many cases of English-speaking parents here who delayed teaching English reading until age 6, only to find that their kid now had no interest in doing so--he or she could already easily read Japanese children's books (thanks to furigana), and this sapped their motivation for mastering the much more difficult English alphabet system.