So when did all this virtue signaling start in media (movies,TV shows, comics and video games)?

I think the bulk of it started last year. Now almost every commercial shows it and some situation comedies.  Who knows .

This "agenda" started about 5 years ago and is not aimed at grownups. It's aimed at children and young teens as a method of brainwashing.

The people who favor that term always seem to use it as a euphemism for not being an asshole.

And it seems that these are usually the very same people who hold views about economics, or civil rights, or any number of other topics, which center around shows of individual virtue for their own sake, complimented by punishment for its own sake, even if they result in bad outcomes in the aggregate, and therefore can't be considered virtuous in any meaningful sense.

My goal here is not to root out hypocrisy, which I don't really give much of a sh­it about, but just to suggest that you try neither to be an asshole nor to hold views that are stupid.

The entertainment BUSINESS is just that - business. They do what makes money. Frozen 2 just broke records for Thanksgiving weekends. Disney executives were astonished when the first Frozen movie made so much money- they had no idea that girls would connect so much with a story about two sisters. So they made a second movie to capitalize on that-- and it worked. But don't worry- they'll keep pumping out action adventure movies with ridiculous male characters for you. 

it started in 1999