It's a flashback to the invention of the fusion drive that allowed humanity to colonize the belts and outer planets, no less. It took place over a century before the events of the series, which itself is estimated to be around 2350 AD [1]. Clearly they've had plenty of time to not only work out the kinks but integrate the technology into society at a level far beyond what we see today with kids and their smartphones. The rock hoppers likely start developing the muscle memory needed to survive in space by grade school.

[1] https://expanse.fandom.com/wiki/Timeline_and_chronology