"Default" makes it sound like a deliberate decision or setting, but that is not how these models work. But I guess it would be trivial to actually make a setting to autmatically add specific terms (gender, race, style, ...) to all prompts if that is a desired feature

That goes back to the data available in the crawler which is mostly white because the english internet is mostly white. If they trained with a different language the default person would the color most often found in that language. For example using a Chinese search engine's data for training would default the images to Chinese people.

Most people represented in photos are younger. Same story.

The problematic issue is the media has morphed reality with unreal images of people/families that don't match society so unreal expectations make people think that having white people generated from a white dataset is problematic.