The answer is obvious, isn't it? Let's see if your obvious answer is the same as ours.
One view is that by the age of about five we've learned more or less all there is to know about the grammar of our language.
We no longer say things like "Mummy not put hat on" and we can say sophisticated things like "Mum, I don't think I want to go through with this!". (A lovely example from Jean Peccei's Child Language - a four-year-old about to start "big school".) According to this view, a five-year-old has a mature grammar, but can't exploit it because of lack of brain-power. She needs a well-oiled and big working memory to hold a grown-up sentence, so she has to cut her grammatical coat to fit her mental cloth.
Plus, the mechanics of writing impose their own special demands which explain why written grammar lags so far behind spoken at this age.
Another view is that grammars go on growing throughout the school years, and indeed into adulthood. A five-year-old is a small person with a small grammar, and as she grows in stature she also grows in grammar - and maybe at about the same rate, with a big spurt at key stage 3.
If this view is right, the growth that we hope to see in writing is a product of two parallel developments: growth of brain-power, just as in the first theory, but also growth of grammar.
Does this theoretical dispute matter? We think it's absolutely fundamental.
How you view your pupils' grammatical development will affect how you teach them. So it really matters who's right.
What's the answer?
Regular readers won't be surprised to learn that we think grammar goes on growing; after all, there wouldn't be much point in writing a column about the writer's toolkit if we thought every five-year-old was already fully supplied with it, would there?
All of which places more emphasis, and responsibility, on the teacher. Our interventions can be very powerful - nudging pupils to extend their grammar, to explore new structures and, crucially, to reflect on their own language strengths and weaknesses.
If grammatical development was fixed, we might as well not bother being involved at all. Fortunately, the teacher's role is crucial.
Richard Hudson is professor of linguistics at University College, London Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk