Not sure when this happened, but I would like to prevent and stop it from continuing. Any ideas?

 

We can try spaying a non yellowing, uv resistant clear coat. Wet sand between coats. If you do it just right it will look like new. If not it will just look a little worse, but on the bright side they won't ever yellow or fade.

 

I've got similar on my 2010. Been watching various Headlight restoration videos on YouTube and am about to give it a try... On the Wife's Tahoe 1st.
Kits are relatively inexpensive but you can probably build your own (similar materials) cheaper and then, as Screwed05 stated, finish with some non yellowing, uv resistant clear coat.

 

I'd get 2K clear if you plan to try to paint them.

 

You can use a headlight restoration kit if they get really bad. Its time consuming but pretty easy.

 

mine have been doing that starting this past summer. i haven't done anything to it yet but i did the restoration on my previous vehicle. I used the megiuars kit and it worked really well. however it got bad again after about 4-5 months. what you need is something to protect from the UV rays. I've seen some videos of either using spraymax 2k clear coat or 3M clear vinyl wrap.

if that's the only spot - then i don't think you need to go with the whole headlight restoration. I'd try the 3M clear vinyl wrap.