Never had problems with plastic becoming brittle apart with some hong kong cheapies, and I don't even think they're made of polietylene.
But some of my esci figures (later issues) exudate a sort of oil, and someone told me some time ago that it's the first step towards brittleness - the additive that makes plastic soft starts to "bleed out" of plastic and then the figures become brittle.
Anyway I always keep my figures into boxes, away from direct light and if possible away from hot and cold, so I hope I won't have any problem for many other years.
Apart from that, is it possible that some chemicals used to degrease figures before painting may speed up the decaying of polyethylene? Because I rarely degrease my figures, and in 20+ years of collecting I never had brittleness problems. Maybe it is related somehow.