I'm interested in possibly buying a used MacBook Pro. The one I am looking at has the El Capitan 10.11.1 OS. I'm wondering how long Apple supports their software or provides updates for it? Basically, how long could I expect to be able to use the potential laptop before it is no longer supported by Apple, and/or is insecure or obsolete?
That question is more complicated that just what the current macOS that is one the computer. It will also depend on the specific Mac model you are considering.
The first aspect is the active support for the macOS. Apple's current policy is that they support the current macOS version (which right now is Sierra, aka 10.12.x) and the two previous versions (which right now would be El Capitan, aka 10.11.x, and Yosemite, aka 10.10.x). The current version will get security updates, bug fixes and potentially some new features during support. The previous two versions will just get security updates and bug fixes. And since Apple tends to release a new macOS version each year, that effectively means that each macOS version will get about 3 years of support. Apple tends to release the new macOS in the Fall. So, that means that sometime this fall, it is likely that macOS 10.13.x (or whatever version number they give it) will be released. When it does, then Apple will likely drop support for Yosemite, aka 10.10.x.
The other issue is what versions of the macOS the Mac can run. As time goes by, more older Macs are dropped from support in terms of being able to run the latest version of the macOS. For example, if the Mac you are looking at is old enough, it might be maxed out at El Capitan, aka 10.11.x (i.e. it is not officially supported to run Sierra, aka 10.12.x). If so, then there is effectively about one and half years left before it will only be able to run a version of the macOS that gets active security updates. Otherwise, it might be able to run Sierra, aka 10.12.x, (but the current owner just has not installed it), but then maybe it is old enough that it will not be supported to run the new version of the macOS that comes out in the fall. Unfortunately, no one but Apple knows which Mac models it might not support for the new macOS until they release it or at least announce it (they likely will announce the new macOS at their developer's conference that starts in a week...that is what they have done in past years). Generally, the newer the model is, the less likely it will not be supported for the next macOS version. If it is a model that will not have support to run the new version, then that means you would have about 2.5 years before you would lose active update support (i.e. basically when Sierra, aka 10.12.x, is no longer supported).
So, without knowing the model of the Mac, I cannot say for sure. You would have at least about 1.5 years as it currently has El Capitan on it, so we know it is supported to run El Capitan. If you tell me what Mac model it is, then I can likely be more detailed. And to give you an idea of how best to describe the model, I am using what is referred to as a Mid 2014 15" Retina MacBook Pro with dedicated graphics.
To give you an idea of where things stand right now, here are the requirements for Sierra:https://support.apple.com/kb/sp742?locale=en_US
You will note that any MacBook Pro model Mid 2010 or newer can run Sierra officially. Thus, as long as the MBP you are looking at is at least a Mid 2010 model or newer, it can then run Sierra. Which then means you would have at least 2.5 years or so before you would lose active update support for the macOS...and could be much longer if the model is significant newer. The most likely next models to "drop off" the list that can run the upcoming new version would be the Mid 2010 models and maybe some or all the 2011 models...and potentially even some of the 2012 models...but again we will not know until Apple either announces the new version (potentially in a week or so) or even until it is actually released (which will likely be this fall).
Edit: I forgot to mention that just because the Mac model can run a particular macOS version officially, it does not mean it can use all the features of that macOS version. Take a look that the link and you will see for example that you need a 2012 or newer model MBP to use the Handoff, Instant Hotspots, and Universal Clipboard features in Sierra. There are other limitation if you look at the link.
Edited by smax013, 27 May 2017 - 10:38 PM.