I firmly believe it's mostly the sun that does the damage.
My TT is a classic case. We don't get the sun here that they do in places like Az or S. Ca. So my 10 y/o graphics are in perfect condition with the exception of the front. While the back and sides are well protected, the front of my TT gets the afternoon sun every day. Those decals faded within a couple of years and started trying to peel after about 5 years. A couple of years ago I had the front re-painted because not only were the decals shot but the gel-coat was going away as well.
Oh and I wax it at least once or twice a years - decals included.
Here's what the look like after ten years:
So the longer you can keep the sun off it the better.