Definitely the term after they turn 5.
DS went at 2 weeks after 4 though, and he's absolutely thriving - the first half term was basically playgroup (handy the weather stays good so they could play outside) - he did full days, but some kids were too tired and just did half days so they were very flexible. He only started reading books after half term and he's already doing so well. Bounds in every day, absolutely loves it.
Make a call on your own child when the time comes - but remember it's not all sat down in rows these days, it's much more fun (they have lego, toy cars, bikes, painting - plus a room full of friends)