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.

It's an aside but age 25 being full development of the brain is a pop science myth