There are extra hazards associated with turning when close to the ground, so traditionally 500 ft is the lowest height to commence a climbing turn after take off for ab initio students.
Notwithstanding any local noise abatement procedures (don't we love them!) it may be sensible to extend upwind (by delaying the turn after take off) to provise separation from traffic ahead whic is also in the circuit. Not much point in turning at 500 ft and following them all the way round the circuit only to have to go around because you are too close on final!
The issue of bomber type circuits which Beagle touched on is a perennial problem where I fly with certain pilots and organisations. The practice of extending downwind for separation should be avoided, the remedy being to go around from the normal base leg (maintaining circuit height) - this means that the a/c behind can, hopefully, fly a normal circuit. Sadly this is rarely taught or practised and one often sees two or three a/c on base leg or even final and no a/c in the upwind part of the circuit.