My friend adopted his step daughter when she was an adult.
The sperm donor was a terrible drunk and the girl felt it was very important that the guy who actually helped her mother raise her become her legal parent. It was largely symbolic but the girl felt very strongly about it.
Both of my sister’s kids from her first marriage, on their 18th birthdays, changed their last name to that of their stepdad who actually raised them. Sperm donor would not permit adoption or name change. Don’t think there was any adoption involved at 18.