On such a short time scale, it wouldn't matter the least
You could say the same about dwarves having kids; it takes 12 years for them to grow up, so why bother?
The only reason it "doesn't matter" is because the game is damn near forced to a shorter timescale than it should ideally have. Dwarf Fortress just doesn't yet have enough outside-of-fortress interaction for long-term fortresses to really have many long-term goals except for the most insane of megaconstructions. Due to this and some other quirks (such as the ridiculously high skill gain rate, especially for children and their social skills for instance), there's just not much of a reason to play a multigenerational fortress.
Now, since multigenerational fortresses are IDEALLY a good thing, and will be at some point in the future, I don't see why long-term poisoning is such a bad idea. Hell, if it could somehow be incorporated into worldgen, that would make it useful in the shorter term, but I'm not sure how it could be.
And yeah, a lot of poisoning wouldn't manifest only after decades; some environmental hazards could harm you over much shorter periods of time, especially if the harm involved is lesser and there's a LOT of exposure going on.
This is easily combined with two other suggestions I've made/seen:
- More toxins! Some of this seems to be going in slowly, but I'd love asbestos and mercury to be in the game, especially since asbestos is damn useful as well.
- Mining should create dust. Maybe you create less if you're a better miner, but hey, splitting cinnabar into chunks and moving it around should make at least some environmental hazard.