I can’t speak to the second half of your comment, but it’s worth pointing out that 31 corresponds with a software engineer who received a BA/BS in four years after high school, started working and hit senior at 3-5 years (a lot of us). That gives a couple years of wiggle room to lead projects after that too.
many senior scientists are around 30-35 years old (by that time they have completed grad school and postdoc and are starting to get their first grants). And in nuclear physics most of these folks were young but had worked in key labs and their bosses were advisors on the project.
