I have trex on my house, that was there when I bought it. The back porch was all gray trex, not sure how hold, but it gets bombarded by sun. The color was still good, but it was weak from all the UV exposure it seems to me - it would flex and bend between supports.
The front steps are made of the same stuff, about the same age, but it doesnt get hit with direct sunlight hardly at all - and they seem much sturdier and dont flex at all.
I am not sure how old it is, or if they have improved the stuff, but replacing the trex in the back of the house with stained redwood was one of the best upgrades we have made to the house so far.