I noticed that my 17 year old Selberg "Max" puppet has began to change its eye color. From white to yellow. Is there anything I can do? Has this happened to your older figures?

Some (but not all) epoxy laminates will exhibit this problem. Back in the early '80s there was a similar problem on a couple of figures I had built during the late 1960s/early 1970s, and it turned out to be an issue with the particular laminate I was using on the eyeballs in my early years of dummy building.
In recent years I've seen a similar issue on other figures made by a variety of makers, and have been hired to remedy the situation a number of times over the years.

If the yellowing gets __really__ bad, probably the best thing to do is to remove the eyeballs and repaint the scleral area without re-laminating.

The master, Mr. Semok is correct. Epoxies will yellow over time.
There is nothing that can be done to reverse the process, but you can redo it and add preventatives.
I often use an automotive clear coat to 'wet out' and gloss up the eye.. It sprays on beautifully.. But it's expensive and it has a short shelf life once you add the catalyst.
It won't yellow though - as it is built for use in the outdoors and UV won't yellow it.
Personally, I kind of like the yellowing of the eyes.. It's more lifelike and creepy.. But not everybody is going for that

Lou, you need to have Nacho checked... could be Hepatitis B

You can have a new pair of eyes installed... but expect to spend a lot of money for the repair and depending on who will do the job, it might also take time before the figure returns to you.