Quora User's answer to How did you change between the ages of 18 and 25? One might call you a "young adult" or if you want to try to get clever, a "proto-adult. their twenties, and girls who were full grown, had their period, and completed. Oct 30, - The phase from the end of puberty (18 to 20) until full adulthood (around 25) is called "yound adult" in science. The exact ages of course vary.