There are two large, about 35 yo Opuntia engelmanii in my yard in Phoenix Arizona USA. Both recently flowered extensively and the fruits are now maturing. I've lived here about 6 months. There is no irrigation in this area of the yard, and I have not watered them (ever). Rainfall last winter was about 1/3 of normal and the previous monsoon season had little rain. So it's been dry even by local standards.
I noticed this week that some pads of one of the plants are turning yellow. In this picture you can see the yellowing, which was not present a month ago. The other plant, only a few feet away, is nearly twice the size and shows no yellowing.
Is this a problem? I could be convinced that this is part of the normal aging of the plant -- this species gets woody near the base. But I also know cacti can get into trouble beyond the point of salvage quickly. I see plenty of cactus turning yellow/rotting questions on this site but they seem to all be about potted cactus, so they don't really apply.