> Among those aged 25 to 44, a group we call “early adults"

A 40 year old is federally protected from age discrimination (for being too old) and can have children which are legally adults. No one is calling this age group "early adults".

The "we" in the article does not include you - it refers to the authors who are mortality researchers and looking to differentiate between different groups of adults.

I discriminate adulthood 2 ways:

"legal"/"federally protected" adulthood - 21 - can vote, drink, etc.

"biological" adulthood - brain is not fully developed until ~25, and takes a few years to "activate/realize" those "powers", so ~30 for "actual, mature" adult processes to start taking place. ~40 seems appropriate within that as an outer boundary of "early" adulthood.