If you are wondering on the 2019 and 2020 things that people are talking about, that's when Microsoft ends Windows 7 support. Microsoft supports all OSes for a minimum of 10 years from the date they are released. So barring any extensions (which happen but are rare) support ends January 14 2020. After that date it will receive no security or other updates. That means, of course, that using it online becomes dangerous because sooner or later a security flaw will be found and it won't be patched. Also when MS discontinues support for an OS, hardware and software manufacturers basically all do as well, and won't take support calls for you if you are running it.
However as a practical matter support from 3rd parties often dies out before the actual end date of the OS. Vista is still supported by MS and will be until 2017, but it is very common to see things not support it anymore. That is beginning to happen with 7 as well. We have gotten some new computers at work from Dell and Lenovo that do not have 7 drivers available, they are 8 and up only. Newer CPUs also need newer OSes as architecture differences aren't supported on older ones.
So while you can keep an OS for awhile, keeping it forever is more or less unfeasible unless you wish to operate a computer disconnected from the Internet and never upgrade any hardware or software.