When did people begin starting their sentences with the word, "So"?

It is perfectly acceptable when it is used to mean the same thing as 'thus', or 'therefore', in a sentence that describes the results of a previously-detailed explanation.

But when it is used like this: "So, I was going to the store and...." You are correct.

I've heard it from time to time over the years, but not as the disgusting linguistic tic that "like" has become and still is. And besides, it is valid conversational usage. If people you know are using it to start sentences as often as teenage girls use the word like, then these people have mental issues. Such usage is not known to be a meme at this time. Thanks for asking, though.

The word "so" has many reasons.

So as a conjunction means - for that reason.

As an adverb - in the manner indicated. In the same way, therefore.

As a pronoun - the same or approximately.

As an abbreviation - south or southern.

So (lol) to answer your question I think it's cuz people often use "well" "and" "but" "or" and "so" cuz they're all connecting words in english literature