1-if someone has ruined his life with illegal addiction, is 70 and maybe near death in a prison, is it wrong when they invite him to tell the youth about the result of such actions and say:"be careful not to end like me."?
No, it is not wrong. Each situation is not the same, but such a thing is not necessarily a bad course of action. His experience can be very convincing to others.
2-If I am in a sin (lying, backnitting etc.) and a child comes and asks if these are sins or not, should I say no, because I am a sinner myself?
No, you should tell the child the truth.
3-If I am addicted to a not so much catholic friendly channel, and see a child approaching TV trying to turn on that particular program, should I let him do it and wait till I am purified, and then do sth about the child, or I should snatch remote control from his hands as soon as I can?
Presuming it is your house, you have some say, etc., you should not allow the child to expose himself to the danger, even if you are stupid enough to do so habitually.
For example, just because I am a regular customer at a brothel does not mean I should not try to persuade a young man to NOT go. Similarly, if I do nothing to dissuade him, or I actually encourage him, I am guilty by silent consent, positive assistance, encouragement, etc.