Changes in the early Barbie

When I first started collecting Barbie I learned that the first Barbie was made with different plastic then the later dolls.  Earlier plastic can have numerous problems.  Barbie 1 - 3 can had problems with plastic.  The #1 Barbie, #2 Barbie and #3 Barbie can be sickly white.  The #4 Barbie had a new plastic, that does not turn white. but some #4 Barbie's can have limbs that are sickly white. 

Now years later after I moved from Texas to New York.  I did not unpack my Barbie items when I first.  I knew what they were and they were packed well, so I just kept them in the box for safety.  I recently started to unpack them.  I noticed that my first year bubblecut Barbie now has very tan legs. 

So now after 50+ years do the #4 Barbie's also have a plastic problem.  Did it just take that long to come out? Does this mean that the plastic will continue to age unexpectedly.