>>680514This is untrue. It's a belief based on the normalization of bras, which change posture and cause the suspensory ligaments of the breasts to atrophy. Think: what did women do before bras were invented or became commonplace? Were women simply defective for all those generations before the modern world developed a solution? Consider the ideology that goes into the kind of thing you said.
They start girls young because they don't want them to get used to natural posture and for their connective tissue to strengthen naturally as they develop. It's like trying to walk normally if you've used crutches your whole life. By redirecting the forces to a different part of the body, bras force a change in posture (and which muscles get used to bearing the weight). By lifting the breasts from their natural position, they take all the tension off the ligaments internal to the breasts, which causes atrophy.
Someone accustomed to wearing a bra taking it off will feel uncomfortable because the body adjusts to wearing one. And in the long-term the atrophy to the supportive ligaments can increase the chance of injury when the bra is off, because the ligaments have not been allowed to strengthen enough to support the weight. This is what causes sagging - the supportive tissue (which can be strengthened like any ligaments) accumulate injury over time. Transitioning to not wearing a bra in some cases actually results in natural lift because the ligaments begin to regain their natural strength. That also can help prevent sagging (injury) in the future.
Worse, there is some evidence suggesting that bras may be linked to breast cancer, although this is hard to study because of the variety of shapes and the mechanism being theorized to be impingement of the lymphatic system. Normally your lymphatic system clears out cancerous cells early on when they appear, but if the lymph ducts become obstructed they may not be able to remove the cancerous cells quickly enough, and tumors may form, eventually becoming cancerous.