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@duci9y You can get Android to run on older devices, as demonstrated by plenty of YouTube videos (search "iDroid" for some examples). It's very buggy, but it boots up and you can tap things. See this guide for tips people figured out to get it to work better: idroidproject.org/wiki/User_Manual
@big_smile You can switch between different versions of iOS in limited circumstances, because Apple's firmware verification process is flawed. This is easy on iPhone 4 and earlier devices since they have bootrom exploits - you just have to save Apple's iOS version signature hashes (SHSH blobs) while they're signing that version for your device, and then replay those signatures later. This is harder on newer devices like the iPad 3rd generation. This guide has more details: jailbreakqa.com/questions/32462/… (I help maintain it).
See the comment here quoting MuscleNerd for technical details on bricking possibilities for various devices. The reports I've seen of bricking due to jailbreaking are pretty much indistinguishable from the statistical likelihood of hardware problems, especially problems with the buttons (which would prevent a device from being able to enter DFU mode).
As already commented on the other answer, it's basically impossible to brick your device just by jailbreaking it. If something goes wrong, you can put it into DFU mode and restore it with iTunes.
