Yeah, but that's the traditional things that are arbitrary. Those decisions weren't really made by scientific consensus, but more by societal needs and norms. In older times you were also considered an adult earlier because that benefitted society at the time.
As we learn more, and as society and its issues changes, so do these agreed upon lines. It's less shameful to continue living at home into older ages because housing markets are harder now, and the brain development thing may not really affect work and the like, but some are arguing that it changes the effect of drugs/alcohol/trauma/life long decisions etc.
Whether this could change how we decide if those ages should affect the items you mention are yet to be seen, but keep in mind stuff like Insurance companies finding people under 25 more risky than above 25. Some industry could argue (probably unsuccessfully) that they may need to move it to 32, or people may start suggesting marriage etc should be later than the 20s, etc.