The next is that Dumbledore would’ve been in his mid-forties in the 1920s, so (factoring in Hollywood’s typically flexible view on actor/character ages) we’re assuming they’ll cast someone within the range of late thirties to early fifties.
The first is that we’re going to assume that whoever plays Dumbledore will be from somewhere in the United Kingdom.
Dumbledore is British, so hopefully he’ll be played by a British actor.
That’s yet to be confirmed, but we’re going to ride with that assumption.