I too was something of a precocious child mentally/emotionally. I read a lot of novels far outside my age level. I would speak to adults like I was on the same intellectual level as them. I didn't like certain kid movies as a child - ones that were highly slapstick never engaged me. I hated Home Alone & Beethoven (the dog movie), both of which were slapstick heavy kid movies that came out when I was a kid.
As an adult, I'd say I am childlike in the sense that I still have a really active imagination. I can play with kids like a kid still. I'm not a big fan of kids, but when I interact with them they tend to like me because I can play like they do still. I'm capable of that silly, ludicrous make-believe still.
I also tend to like playful, colorful, whimsical things, from clothes to music to movies. However, I also like refined, elegant, complex things, and I have since I was a child. It does become a mix of childlike/sage, which you often hear used to describe to both INFPs & INFJs.
Really, I would say I am a "perpetual teenager" more than a kid though, and that's something I see in other INFPs (ISFPs too though). There that's moodiness associated with teens that hangs on into adulthood, although it's far more subtle. There's also that obsession with independence, identity, and self-expression - themes that preoccupy many teenagers.
A love of novelty can also seem childlike because old people tend to get nostalgic, whereas young people tend to embrace the new. From what I can tell, many NFs tend to be on the childlike end of that spectrum. I know that I tend to be far more future-oriented & embracing of the new over nostalgic.
