Dude, I see that claim that kids with high IQ are super mature made all over the place, but have seen slim evidence of it IRL. They're kids who like fart jokes and wriggle and hit their friends just like any other, even if they have conversations about esoteric subjects (which display a very age-appropriate lack of wisdom).
I will preface this with the hallowed caveat: when you see one gifted kid, you've seen one gifted kid. And I'll add that I'm a middle-aged giftie who still likes wriggling and fart jokes, so I don't necessarily identify those traits as part of maturity (in fact, I believe I've seen an article recently about a study that correlated adult playfulness with giftedness).
That said, the common mismatches I've seen are:
- Interest in subject areas ahead of age peers (which can be anything: science, literature, current events, entertainment, etc).
- Sense of social justice.
- Capacity for sharing and other pro-social behaviors.
- Self control.
This is all relative. We know that an 8yo is still going to be having impulse control issues, we also expect that there has been significant improvement from when that child was 5. When you put 8yos and 5yos together in a room, you should note the differences. If you put your 5yo in that room, and note that he behaves more like the 8yos, he gravitates towards them, and the 8yos are more accepting of him than any of the other 5yos, that's a pretty good sign of a mismatch.
I see that sort of phenomenon with my DD10 in mixed-age groups ALL. THE. TIME.
