If you feel your child is getting behind at school, and you think they need extra support, then early intervention can help them to catch up or keep up. In cases such as this, the earlier you start the better. Of course, if you feel that they no longer need tuition, then you can always stop. I frequently tell clients that the purpose of tuition is to get their children off tuition!
Some clients have a particular goal in mind. In many cases it is to help their children prepare for Key Stage 2 SATs. If this is your requirement then I would recommend starting tuition in Year 5 (especially if the student isn't strong in maths). This enables us to focus on the areas your son or daughter needs to work on early, so that by Year 6 we can really get to grips with the Year 6 curriculum objectives.
Of course, if SATs success is the reason you are looking for a tutor and the student is already in Year 6 then the earlier they start tuition the better, but certainly from January of the year they are due to take the tests in order for it to have a real impact.