It is indeed the case that they yellow.
The 3m product does it far less then then the cheaper films, but yes, it does yellow after about 2 to 3 years, depending on maintanence done ect. But that is the case with any paint protection film.
What makes it actually look worse, is that when you do a partial bonnet, the line where it was cut actually picks up dirt and stuff, and then its even more noticable.
So your answer is, instead of doing a partial bonnet and fenders, do full, then it is not noticeable at all, and is good for the lifetime of the film, which is warenteed up to 5 years.