The casting of ; in one scene, McGonagall was asked how long she’d taught at Hogwarts. “Thirty-nine years,” McGonagall replied. Given the Harry Potter stories are set in the ’90s, that means she began teaching at Hogwarts in 1956.
But J.K. Rowling has spent the last few years fleshing out the Harry Potter universe on her Pottermore website. There’s a sizable entry for McGonagall, revealing her Scottish “half-blood” heritage, an early heartbreak when she fell in love with a Muggle, and even how she began her teaching career. According to Pottermore, McGonagall graduated from Hogwarts with top marks and briefly worked for the Ministry of Magic. Disgusted by the anti-Muggle prejudice she encountered in the Ministry, she quickly decided she’d had enough after 2 years and applied for a teaching job at Hogwarts. Between Harry Potter and the Order of the Phoenix and the Pottermore website, then, it’s possible to work out that McGonagall was born in 1935. She shouldn’t even be alive in 1927, let alone already in her ’30s.
McGonagall’s backstory on the Pottermore site is remarkably detailed, which suggests Rowling never really expected to revisit the character. And that, fundamentally, is the problem with the entire Fantastic Beasts film series; these are movies Rowling never expected to script. Had she been dealing with entirely original characters, she’d would have avoided these kinds of contradictions, but every time she decides it’s time to use a familiar name, she’s forced to rework a character into the story of Newt Scamander, Gellert Grindelwald, and Albus Dumbledore. Sometimes that will inevitably mean that their history changes. In the case of Professor McGonagall, it appears to mean she’s significantly older than Rowling originally intended.
Nobody yet knows what McGonagall’s role will be in Fantastic Beasts: The Crimes of Grindelwald. All this is written assuming that she’s a teacher at Hogwarts; it’s a reasonable assumption, but there is just one other alternative. It’s just about within the bounds of possibility that, at some point in the film, Albus Dumbledore will experience of his personal future – and perhaps glimpse Minerva McGonagall, a woman who’s yet to even be born, at his side. That could conceivably happen, especially given dialogue in the Fantastic Beasts 2 trailers has already suggested Dumbledore’s old flame Grindelwald has been having visions of the future; but it does seem rather unlikely. There’s no evidence whatsoever that Dumbledore has ever been a Seer, a subject that would surely have come up at some time in the Harry Potter series. So, sadly, it’s far more likely that J.K. Rowling has just made a significant continuity error.