Lol! I read the same article and had the same thoughts at the ending! It's because they're bloody useless at looking after themselves, clearly! They're happy with Mummy doing their laundry and cooking their meals. It can't be the finances because otherwise men and women would stay at home equally.
I think women leave home earlier because they're more often controlled at home, too. Can't have a sex life, basically. Parents are worried about being left holding the baby. Not so much with sons, because it'll be more like HER parents left holding the baby.
I don't understand why people stay at home in their late twenties/early thirties. I know about housing costs, but it was the same in London in the late Nineties/early 2000s as well. Everyone in my cohort left home and rented rooms in a flatshare. They were wonderful times, and essential for maturing. And if you don't insist on living in an expensive Central London share, you CAN save money while flatsharing.
I think perhaps young people's expectations are higher these days, and they don't want to rent a room in a not-wonderful flat. For my generation, moving to London after uni and flatsharing was a rite of passage.