The Most Cut and Dry is Mac OS - Apple is very straightforward in the lifespan of their products. Most machines are created to last the average consumer-age of PC's - yeah, Planned Obsolescence - which is about 3-5 years. This standard of age for computers means that Apple usually drops support for hardware about 5 years old or older. The idea is that you take your 5 year old device to an Apple Store and trade it in on a shiny new one with the latest operating system to use for the next 3-5 years. It's a system, not a system I like, but a system none-the-less. The upside is that you tend to end up with hardware that outlasts the software, and that means that you might "Save" a little bit. The bad though is Apple devices cost up to 4x the cost of a simliar PC when new, and so that "save" might still be out-edged by the savings you'd get buying a similiar brand new Windows-based PC product without having to trade in your old mac (ie drag another computer to the store with you). Also, they tend to cut features ( such as 32-bit support or using your iMac as a secondary monitor for a second device) from their newer O/S versions which can bea real problem. A lot of the time, when it comes to older Macs that are out of support, I keep the last supported O/S version on a second partition on the Mac, and then run LInux in tandem so I can get the most out of the machine. But that's my use-case, yours may differ. In that case, MacOS gets reduced down to use for GarageBand and light Internet activities with the latest supported browser (which get more limited if the browser loses support), while Linux takes up the slack MacOS Can't.

Microsoft Windows is a bit More Ambiguous, but Not Much - Microsoft is the funkiest one of the lot. See, they don't adhere to the "3 to 5 years" thing as much as Apple does, but they do have a finite amount of time that their Operating Systems are supported - which seems to get longer in the tooth with each good version because it seems, after they get enough bad press, they start listening to their user-base again, for awhile. It's this cycle that's the typical cycle of Microsoft Product releases. One is crappy, experimental, and has a lot of problems, only to be replaced by something far more stable and in-line with what their user base, and the public, wants. So the common lifecycle is about up to 10 years with a few extra tacked on if it's a especially great product such as Windows 3.1x, Windows 2000 Pro, Windows 98 Se, Windows XP, or Windows 7. If it's terrible, Microsoft might cut support as early as 5 years like Apple does (ie Vista or Me). Thing is though, at about 8-9 years old is when Microsoft user experience starts to become really crap, because they keep tacking on more "bloatware" to the O/S (un-necessary widgets, features, and programs that needlessly eat up RAM, disk space, and CPU time) that slows the machine down. These days, personally, I don't run Windows as much because WINE, Lutrix, and VMs on VirtualBox tend to replace a lot of that stuff effectively enough that I don't need to have a full Windows installation to do what I want to do with Windows, plus I rely on Open Source more these days. So most of my would-be Windows machines run LInux Mint and have the aforementioned setup on them which works out fine for me. But it's all about YOUR use case.

Linux has the longest lifecycle and part of it is the ambiguity of what Linux truly is due to being a part of multiple distributions as a chassis for multiple "operating systems" in a way. Upgrading Linux is different than commercial OSes in that they don't try to push you to buy new hardware for your installation, and how much it might affect the hardware you are using, may depending big time on what distribution you are using. For example, a co-worker turned me onto Pinguy Linux, which looks a lot like MacOS. Thing is though, to have that kind of behavior in Linux means a lot of background stuff with a lot of CPU and RAM usage compared to a lighter weight distribution - such as the Linux Mint x64 distro I'm typing this on now. Pinguy wasn't happy on anything 7+ years old, but Linux Mint runs great even on my 15 year old Mac Mini (for the most part, for regular day-to-day stuff) and runs all current, safe software. Plus they tend to give you a smoother upgrade path if you're technically knowledgable. The last 3-4 mint version upgrades I just let my PC install them in the background while I was running around the house doing other things. On a system 10 years old or younger though, Linux runs great. I even have a 8 year old Core i7 laptop that looks like hell that I'm using as my main laptop gaming rig - dead CMOS battery and all.

However, it's not just the operating system, it's the support of the HARDWARE that's an issue. First off, a lot of 2000's era and early 2010's era hardware had security issues due to Spectre and Meltdown viruses which exploited the Core 2 series CPUs at the hardware level. These NEED to be patched with the latest Firmware update that SPENDS some CPU speed on protecting the computer from unwanted attacks. Lucky me, I don't have any Core2 machines anymore (well, except my Mac Mini really). All of mine are i3/i5/i7 machines made since the 2nd generation, with the oldest being the HP EliteBook.

One tremendous issue is MAC hardware. Because Apple is a closed architecture, and tends to hold a death grip on who can and cannot access the tech specs for their hardware, Microsoft Windows or Linux might not always work as expected. Exhibit A is my Late 2015 iMac 21.5" - my main desktop as of this writing. The machine dual boots MacOS and Linxu Mint, and it does so because the audio card does not fully work in Linux Mint. The part that does not work is the external speakers, for some reason the switching mechanism between the headphone jack and the speakers fails to do what it's supposed to do. So most of the time, when I'm using my iMac, I'm using my Line6 HD500 for the audio Device in Linux. It also can't find the "Magic Keyboard" over Bluetooth easily enough to be reasonable without me connecting it via the cable Apple provided, which I need to do anyway so that I can dual-boot the machine (pressing "Option" while booting the MAC so I get the boot option prompt). Anyway, about that Line 6.....