Halliday & Matthiessen (1999: 615-6):
But there are two further developments to come before the ideation base can take the form it has to take if it is to produce discourse of the kind we have been assuming throughout the book; and there is some elapse of time before children take these further steps. The first of these developments is abstractness; the second is (grammatical) metaphor.
General terms are not necessarily abstract; a bird is no more abstract than a pigeon. But some words have referents that are purely abstract — words like cost and clue and habit and tend and strange; they are construing some aspect of our experience, but there is no concrete thing or process with which they can be identified.
Small children simply ignore them, but by the age of about four or five they begin to cope with abstract meanings; in literate societies, this is the time we consider that children are "ready to start school", no doubt because you have to cope with abstractness in meaning in order to be able to learn to read and write (cf. wordings like spell, stand for, beginning of a sentence).
But it is not only the written medium; rather it is the whole world of educational knowledge that demands such abstractness in meaning. Consider examples taken from primary text books such as Some animals rely on their great speed to escape from danger, or The time taken by the earth to rotate once on its own axis is a day.
What happens here is that experience is being reconstrued in order to build up a form of knowledge that is systematically organised and explicit. Children already know that animals run away because they're frightened, and that the sun goes round the earth once in a day; but they have to learn these things over again in a new, more abstract semiotic frame.