alberta is young but this is for all of alberta (they live in the old part of alberta - Lethbridge, but it doesnt really matter in terms of the booking)Might have to do with demographics too. Ontario has a poop ton of old people. Not sure about Alberta or the specific area your parents are from. They may have just run out of 70+ folks to give it to in that particular area earlier than the rest of the province or other provinces.
Even if Ontario distributed their entire current supply we would not be on 60+ year olds.
apparently they way they are doing it is
they are doing doing 50-75 year olds now
it goes in reverse order for booking
so 75 year olds can book on day 1, 74 on day 2, etc
when they actually get the shot is TBD