The female died yesterday, as predicted. 2 females remain, seem healthy for now.
The biggest reason I doubt stress is that it affected 2 shrimp in a way that appeared to more like disease transmission - although I don't know for sure, obviously - and seems to resemble a description of a shrimp disease (which I found by looking in a commercial shrimpery book). The diesease was called "white spot virus complex" or "white spot syndrome baculovirus". I tried google, but didn't get good results to confirm / find more info on the first try, and haven't had time to look more yet. The book gave only the vaguest description, but I intend to look more at this virus to see if it could have been the problem.
I do not use ferts. This is a "shrimp only" tank, dedicated to them. (It's not actually only shrimp - the other inhabitants are 3 banjo catfish, 1 guppy who hitchhiked into the bag with the shrimp as a baby, and an applesnail). I added the applesnail very very recently (he was in another tank, and not doing too hot in there because the fish picked on him). It just occured to me that this could be related to the deaths (although I can't really imagine how - and it could of course be coincidence).
I wish I had a way to guess at age, but I really can't. It could have died of old age - if I hadn't lost 2 shrimp to the same symptoms a day apart. The male came from a LFS, and all the shrimp from this shipment were larger than I normally see in stores - I'd guess around 3 inches, but am poor at estimating sizes. I think that he actually lost size when he molted, but I never measured, and it could be that my perception is off because I keep my shrimp in a larger tank than the LFS.
Interestingly, both shrimp that died displayed a very red/orange coloration, much brighter than their counterparts. I have seen them display this kind of color, but not on a regular / long term basis. Mine tend to be more brown or yellow colored. More interestingly, I read on petshrimp of someone who had the same tendency (bright red and they died shortly after, although their shrimp never sounded sick if I remember correctly).
I know that no matter what conclusion I reach (if I reach one) that it will be based entirely off guesses. If you happen to come across / know of resources on shrimp disease, health, etc, I'd be quite interested. Whenever something like this happens I turn (if I can) to books and try to learn something substantial. The book I mentioned above is "Freshwater Prawn Culture", and deals primarily with the genus Macrobrachium. I am simply assuming that disease would be similar for shrimps in general, especially since I don't know of any research or publications regarding filter feeding species.
Sorry for so much words, thanks for all your help, even if I end up stuck here with no idea what really happened.