Am I committing a sin and is it considered silence regarding the truth if I see some girls talking to young men and going out with them, and they do not accept advice?

Enjoining what is right and forbidding what is wrong is an obligation, and the fundamental principle in it is to offer advice with kindness. It is not permissible to abandon this obligation under the pretext that the advice will not be accepted, as this might be a weakness from Satan. Allah will ask His servant on the Day of Judgment: "What prevented you, when you saw the evil, from forbidding it?" So, whoever remains silent about the truth is indeed silent about the truth. If the advice is repeated and it is highly probable that it will be of no benefit, then there is no harm in abandoning it.