Since you found the answer, why didn't you mention it? It's nice that you gave a citation, but that's a 100-page document that you linked to.
The answer is that if you are single and under 65, then you don't have to file a U.S. tax return if your gross income is less than $10,350.
(As usual with the IRS, nothing is simple. There are reasons you might be compelled to file anyway and possible benefits to file even if you were under the threshold. But you can still give the short answer of $10,350, and a link for the long answer.)
As a practical matter, the IRS has bigger fish to fry. There's so many lower-income people who don't pay income tax, and a few get made an example of, but most fly under the radar indefinitely.
The answer is that if you are single and under 65, then you don't have to file a U.S. tax return if your gross income is less than $10,350.
(As usual with the IRS, nothing is simple. There are reasons you might be compelled to file anyway and possible benefits to file even if you were under the threshold. But you can still give the short answer of $10,350, and a link for the long answer.)