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Any given Mac model, once released for sale to the public, remains virtually unchanged throughout the run of its production until it is replaced by the next, updated model. This is similar to the way car makers market their vehicles. But instead of noting a "date of manufacture" which changes annually for cars, the Mac's "release date" remains the same throughout the production run, which has been known to last a couple years for some Macs.
If you buy a new Mac directly from Apple, you will always be sold the most currently released model.
If you buy a new Mac from any other reseller, you may be offered a model for which production has ended. Though that Mac is still "new, in the box", it may have a release date of a year or more earlier, and you will likely have paid a little less for it than if it were the current model.
If you shop for a new Mac based on price alone and anywhere but Apple directly, you probably won't be buying the current release. But usually you can still expect to get a solid eight to ten years of productive life from the computer.