Modern vehicles use polycarbonate fronted headlight units allowing car designers to create interesting and unique headlight shapes and forms. This was not possible (or incredibly difficult) using the previous glass lens technology.
Whilst being very tough and malleable the problem with polycarbonate is that it becomes cloudy as the surface oxidises. To avoid this car makers apply a spray coating to seal the surface. However, over time this coating can breakdown when exposed to UV rays. In most countries this isn't too much of a problem as on average it will take longer than the car's expected life for the coating to fail, but due to the harsh New Zealand sun it is a particular problem here. Typically most vehicles exposed to 6 or 7+ years of the New Zealand sun will show signs of headlight UV damage.
Once the coating has broken down it ceases to prevent the polycarbonate oxidation and therefore the headlight will become cloudy and opaque.
Headlight yellowing is also a symptom of UV damage. In this case the particular sealant used by the car manufacturer has a characteristic of discolouration as it breaksdown.
Does it matter that my vehicle's headlights are cloudy? Yes - [see here]