I started reading the novel stories when the kids were 3yo and 6yo. Both love them. My 3yo for the drawings mostly.

There are a number of excellent picture book adaptations of stories that have been published too. But, we read those afterwards and obviously they aren’t as enjoyable to me.

Because this is HN: My tradition is to use my Inkpalm 5 and read them with the lights out at bedtime- we pass the reader around to look at pictures.

Not the previous poster, but based on my own experience as a kid and also my kids I'd say age 5 is perfectly good age to introduce the books.

As an adult you pick up on some the more serious themes but as a kid you just enjoy the story and the bit of danger and overcoming and the overall wholesomeness.

Mine started 5/6, but the more recent books are made for kids. The earlier newspaper cartoons are not. Same with TV, the later Japanese/Dutch produced TV series is lovely. The earlier TV series (there are several) are dark as the cupboard under the stairs and the Groke goes postal killing everything in one.

[0] https://www.moomin.com/en/blog/moomin-tv-animations/

While all the books have both humour and darkness, the early books are more whimsical and playful while the later books are more about loneliness, alienation, and loss.