I've never gotten the whole...
While hormones would still mess with you big time, an adult has already gone through that once, and thus has some experience. Plus, a grown-up (mostly) isn't as much of a dumb-ass as they were when they were kids. There are too many exceptions to this, however, as anyone who has so much as gone outside can tell. Your mindset is at least as much your collected experiences as it is the neurochemicals in your system.
Look at it this way, someone with a condition that delays puberty or otherwise lowers the chemical overload normally associated with being a teenager is still going to act their age. Having a smaller and less-developed body isn't going to change that. Another factor at play is how everyone's sense of time (especially for long-term events) is your brain looking at that timeframe as a ratio of everything that it's experienced up to that point. This is why summers seem to last forever when you're in grade school, but seasons fly by as an adult. Someone reincarnated into a 5th grader's body is still going to have those same collected memories, and thus the same outlook on time. There's just no getting around that.
All that said, there's some inconsistency with the MC's age. This chapter said that he was 16 when he was reincarnated, and I think an earlier one said that he was in high school when he died, but an earlier one stated that he was 23 prior to getting trucked.