It prevents the person from experienceing things they are not mentally able to handle...In my humble opinion. It is a tough one though.
In working with challenged individuals, I recall one woman who was mentally on a level with about a 6 year old. There were things she could do for and by herself, but physical growth was something that was difficult for her to deal with so that despite the best and most patient of teachings about the menstrual cycle and periods, she freaked out when she started her period at 14. I don't mean simply freaking out, I mean, totally losing it because she could not comprehend what was happening to her. She was sedated and hospitalized for almost a month but afterward there were severe lapses for her. Ultimately she ended up pregnant at about 19 and again, could not cope and did not understand what having a baby really was. She went into labour on the street, the baby was put into foster care immediately and later adopted out. She spent the rest of her short life having nightmares about what happened and much of that time she was in a mental health facility. I'm sure the decision the parents made was not without a great deal of thought. sammieswife.