Take this with a huge grain of salt as in some of these countries like Slovakia, you don't need to update your residence address for most things. This means that most people in these age groups might show up as living with their parents, while they just haven't updated their address, even if they've moved out.
Also in some countries it's difficult to change your address if you're living in a rental, which also makes measuring this much more difficult
I don't know if the data are taken based on your address or based on polling. But I never change my address to my rental because it's a hassle to do it every couple of years