Professor McGonagall was already getting on with years the the Original HP series.
From a person who has not read Cursed Child yet, I'm only reading it from here
And she's able to live for 19 years, without retiring...
And also Dumbledore is older than dirt,
With Elphias Doge, and Professor something, who supposedly tested Dumbledore on the OWL's (I think, correct me if I'm wrong).
And JKR can simply not be that bad at math(s).
