X-Men: Dark Phoenix (2019)
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Although it hasn't been stated officially, the "X-Men" films appear to operate on what is known as a "floating timeline" or a "sliding timescale". This concept is used to advance the timeline of a work of fiction without rapidly ageing the characters to reflect the passage of time. As such, the characters themselves exist in a "timeless" state while the world around them progresses.
Officially, Mystique is the only character whose lack of aging was explained. In X: First Class (2011), Beast explained that Mystique's cells age at half the rate of a normal human.
A different answer: While not implicitly stated, this movie evidently takes place in an alternate universe. The "Multiverse" is a well-known concept of Marvel. Since things happen in this movie that are absolutely contrary to prior-movie continuity, it can only be explained by the realization this is the X-Men in a different part of the Multiverse. Edit
You only need to watch the main X-Men movies in order to understand "Dark Phoenix". They are, in release order: X-Men (2000), X2 (2003), X-Men: The Last Stand (2006), X: First Class (2011), X-Men: Days of Future Past (2014), X-Men: Apocalypse (2016), and Dark Phoenix (2019).
Second answer: if you are aware of the X-Men as a concept, it is unnecessary to have watched any prior movie. This movie stands apart from all others, evidently taking place in an alternate universe. As such, story lines established by prior movies are irrelevant to this very-different version of the X-Men. Edit
Director Simon Kinberg admitted that he did not take the events of X-Men: Apocalypse (2016) into account when he made Dark Phoenix (2019). In an interview, Kinberg stated: "That Dark Phoenix foreshadowing was troubling to me because Jean's power...is otherworldly. It's the Phoenix Force. It's not just something inside of Jean. Jean discovering her own innate power and having it amplified by something in a more visually interesting way was a great idea for 'Apocalypse' because Jean's finally letting go, but then the notion of her becoming the kind of Phoenix Force that we know didn't exactly make sense."
Second answer: Since it is evident that Dark Phoenix (2019) and X-Men: Apocalypse (2016) take place in alternate universes, what happened in one does not influence the other. Any liberties can be taken in such a production (and in Dark Phoenix (2019), definitely were taken). The explanation to ALL continuity questions is simply: this is an alternate universe. Different story line.
Furthermore, this movie states the Phoenix was drawn to Jean's power meaning Jean is already powerful. It is possible that the flare in Apocalypse was the Phoenix Force locating Jean which may have started its journey to Earth. Edit