Glow plugs, according to what is written by some manufacturers, should normally be replaced between 65-70K miles. They are not routinely shown as a replaceable service item however, as they can work effectively for longer, as yours seem to have done. But the time has now come to change them as when the tips get corroded, metal can drop into the cylinder causing damage. Also, cold starting can be less effective but, in theory, not so much in direct injection diesel engines, as in the C5 above. In such engines, the use of the glowplug, which stays on for a short time after the engine is running, is to improve the combustion of the fuel resulting in less pollution.