Why would white smoke be coming from the engine when it is in idle or at low speed?

It depends on the make/model and not so much as the mileage, but how well the maintenance has been done on the car. If the engine just puffs when taking off from a stop, then it could be the valve seals/guides letting too much oil get into the combustion chamber. It could also be the wrong viscosity of oil. If the engine smokes more like all the time, then you will be having oil ring control problems. Then again, now that the weather is cooler outside, you may see steam coming out of the tailpipe. This would be absolutely normal.