I think it started with the discovery of the microbial world. The idea that millions of foreign agents can inhabit and live inside us is absolutely terrifying.
Addendum: define these words as you wish, if you wish, in your post:
When - a moment or frame of time
Modern- versus ancient. How we are different than a tribe in india, if india had tribes
Age- a period of time unified by an idea or process or approach
Started- the first cause, if you will, of a final eventuality
Your post looks like you pulled it out of a Textbook on "Philosophy".
The question, on the face of it, is ridiculous.
The answers can be no less so.
Any answer is going to be predicated on the situation of the person giving the answer.
Europeans and Euro Civilizations are likely to see it as you do, or electricity or ...
Sub Saharan Africans might look at the question is total askance
West Asians might do the same
East Asians might simply laugh at the oddness of the question ...
An example is "the Age of Enlightenment" was supposed to have been what heralded the American and French Revolutions except that their versionof "Enlightenment" stemmed from two completely different Social and Political "Thrusts" and resulted in two completely different Social and Political "Resuts" and yet both groups frequently consider Their Version of That Incident to be THEIR "Age of Enlightenment".