Non-spoilery Spoiler:
"The Blip" creates way too many plot holes for the couple jokes it gives. E.g. why is the 21 y.o. still in high school? And are there no stat rape laws anymore?
Because they aren't 21. They may have been born 21 years ago - but they haven't matured to an actual 21-year-old. They are still 15-17 years old.