A young man from Hudja wearing a jubaq, the national skirt that Huddic youth have to wear during their entire childhood and adolescence.
The belt is supposed to be slightly tightened around a child's waist at the age of 3, the way that is comfortable for the child, thus at the time it doesn't reduce the waist at all. However, the belt's size must remain the same until the person turns 25 and being worn permanently it forces the innards to shift below the waistline, while the person's stomach and kidneys move closer to the diaphragm. Usually at the age of around 10-12 due to the lack of space, the waistline is already clear of all the organs except the small intestine, the backbone, veins and other minor things.
At the age of 25 the person is considered an adult and is legally allowed to take off the jubaq, so not wearing a jubaq is the main sign of adulthood in Hudja. In case a boy becomes an apprentice of a priest and later a monk, he keeps wearing his jubaq during his entire monkhood as a symbol of innocence.
Okay, maybe I should stop watching the Discovery Channel...
Apparently because of the phrase above many people thought Hudja is a real country. Well, it's just a fantasy country I made up and of course all its traditions are fictional as well.
EDIT: Changed the "maturity" age from 20 to 25 based on the comment: [link]
EDIT 2: A bit of extra details...
The world record is 13 inches on an adult woman. It's incredibly small, probably even smaller than in this drawing, and she started to train when she was already over 40. So it proves that it's at least physically possible without affecting the blood flow and digestion.
But here we have a whole different thing: first of all, it's not technically corset training, since when they first get the belt at the age of 3 it's not cinching their waist at all, but as they grow up the belt shapes their body differently. But since the body grows extremely slowly, it has all the time to adapt without complications that temporary corsetting would cause.
See, every year the body grows just a little bit, the waist grows up to 1 inch a year on a normal person. At the age of 2-3 the child's waist is about 19 inches, at the age of 10-12 it's around 28 inches, at the age of 20 it's about 35 inches or so on an average person. A year is 365 days, meaning that the waist grows up to 0.003 inches a day. Training to reduce the waist by 0.003 inches a day seems like an extremely soft compression actually.