Halliday & Matthiessen (2004: 636):
Unlike interpersonal metaphor, the other type of grammatical metaphor, ideational metaphor, is learned later by children and is not part of the grammar of ordinary, spontaneous conversation that children meet in the home and neighbourhood; rather it is associated with the discourses of education and science, bureaucracy and the law. Children are likely to meet the ideational type of metaphor when they reach the upper levels of primary school; but its full force will only appear when they begin to grapple with the specialised discourses of subject-based secondary education.