This is entirely untrue; but the misconception persists for a very simple reason. Beyond eleven, witches and wizards tend only to manifest accidental magic when they are both stressed and either drunk or very tired, and both types of situations tend to coincide with fuzzy memory. The minority of people in the magical world who are aware of this phenomenon tend towards the groups of people most likely to encounter drunk and/or exhausted people while they are themselves in possession of all of their faculties - that is, bartenders, Healers, and teachers.