Maybe my case is unusual, but the car has always been garage kept. So who ever told you they should be getting brittle may be wrong.  If I was you, I would just use my own judgement and do not replace if not needed.

 

Steve, I don't know if mine was garaged kept all the time, but it was all original when I got it and the bumpers still look good. I do however expect that some time in the future they will become brittle, I hope not, but from what I've heard it will eventually happen.