Stress can certainly take it’s toll. Look at any U.S. President before and after eight years in office. Anakin was horribly burned and spent two decades in essentially a portable iron lung suit. Plus the effects of being on the Dark Side, which certainly didn’t help Palpatine’s looks.
That doesn’t explain why he looks old as a force ghost though.
Hayden Christensen is 40 now (almost the same age as Vader in ANH) and he still looks pretty much like his younger self in ROTS.
If Anakin never fell to the Dark Side, then there is no way he would look like Shaw.