According to the WhatsApp support page, the company currently supports Android 4.1 and newer, which indicates that devices produced after 2013 should continue to work with WhatsApp in the future.
The company said that Google debuted Android 4.0 Ice Cream Sandwich nearly 10 years ago on October 18th, 2011, it's very unlikely that some peoples' phones are running anything lower than Android 4.0.4, which was the final update for Ice Cream Sandwich.
WABetaInfo also points out as a practice, when WhatsApp decides to remove the support for a specific Android or iOS version, its dev team also sees how many users are on that version, and the decision is confirmed when there aren't a lot of users using the obsolete operating system version.
In the case of Android OS 4.0.4 and older versions: WhatsApp has announced that they will no longer support Android phones running those versions after a new WhatsApp beta for Android update.
The same applies to anyone who bought an iPhone before 2016. Users with iOS 10 and above, however, will be able to use the application.