It is unlikely to be printed on the box, but every Chromebook has an "Auto Update Expiration (AUE) Date" after which the operating system is unsupported by Google. […] The authoritative document on the subject is here, where Google explains that it "provides each new hardware platform with 6.5 years of Auto Update support". While 6.5 years sounds reasonable, Google starts the clock ticking "when the first device on the platform is released". The exact definition of a "hardware platform" is not provided, but it presumably relates to the motherboard used. Later models may use the same hardware platform, which means they are not supported for as long. It is nothing to do with the purchase date.
This is unfortunate in so many ways.Buying a Chromebook? Don't forget to check that best-before date