Do they have some kind of clear coating this days?

 

Headlamp covers are generally plastic and any plastic deteriorates in time. The biggest threat is the sun so a good UV protectant will slow down the deterioration and keep the lamps clear. I use a good sealant (same as I use on my paint) and follow up my weekly washes with a spray or "topper" of something like Duragloss AquaWax. Another good choice to use now and then is Chemical Guys Hybrid V7. I use these products not only on the plastic but paint, wheels and even glass. They are really good gloss enhancers and protectors. Here is another product that will help: http://www.autogeek.net/plastik-surface-sealant.html.

If you only plan on keeping your car a few years and it is garaged, chances are you won't have any trouble. But the more exposed to the outside and the longer it is exposed, the more problems you will have and the quicker the problems will show up. The Wolfgang product I linked will prevent this. It is good stuff but it is fairly expensive. I find the AquaWax does just as well, is more versatile and is much cheaper.

 

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I use all chemical guys products on my cars and it keeps them looking new.

 

The "yellowing" on older polycarbonate lenses had to do with UV deteroriation of the old style lacquer coating.

If waxing and sealing makes you happy - or add a DuPont or Xpel film lense protector - do your thing.

For nan-clearcoat - since this type of clearcoat is a bit micro-rough - you can seal it for better adhesion of any quality carnuba wax - search Amazon for Einszett "Glanzwax" (blue top) - under $20 for 4-5 applicaions every 6 months or so - add a affordable spray carnuab on top - easy wipe-on/wipe-off.