Mold is extremely variant in where it shows up due to

* climate (ambient humidity and temperature)

* ventilation (both HVAC systems and habits of leaving windows open etc., but also draftiness in older houses)

I'd say it's primarily a humidity (or specifically walls-at-dew-point) problem, but that is of course highly inter-twined with temperature.

Old houses can be more affected due to the addition of inappropriate modern materials, or for example lacking a bathroom extractor fan.