>66F is ridiculously cold to me...I would find that very uncomfortable and elderly people would be shivering constantly and highly susceptible to respiratory illness.

I know people who live in the Mediterranean and get by with no heating during the winter with indoor and outdoor tempuratures this low or lower, so it seems that one can be conditioned into doing so.

Perhaps it's the presence of more sunlight on average rather than the temperature that makes the difference.

You’re forgetting about humidity. Mediterranean climate has comfortable humidity year-round. Where I live, winter relative humidity is 0% because outdoor humidity is nonexistent from freezing temperatures.