When does a teenager become and adult?

Hi
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I recently posted here
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"instant single parent of my 18 year old son!"
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a number of questions about how to try to handle a teenager:
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* whose values are diametrically different from yours ...
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* who does not respect you
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* who does not understand you
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I am not so good at "talking" and anything you tell him (even the
most minimal thing) could end in a "who are you to be telling me what/
how to do, if you abandoned me! ..." tirade
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If I tell him to get a job to pay for his own things he would yell
you out telling you "you abandoned me once and are doing the same all
over again! ..."
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Which to me doesn't even make logical sense: How could you abandon
someone twice?, let along factually; I never failed caring about him
(I mean sending him money, writing and talking to him about the
importance of school ...)
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Apparently he sees me as his "sending money daddy" and now as an
adult feels "betrayed"
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I think I made the mistake to totally underestimate peoples concerns
partly because I had a hard time being cynical and also because I had
a hard time when I was trying to leave the country (my family would
not help me) so I would have been a hypocritical asshole hadn't I
helped him
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I think I should not back off till he understands basic respect at
the very least as his landlord
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My wife tells me she has had very similar problems with her kids
since they became teens, but there was bonding before they were really
lovely kids and she still has that in her mind. All I see in my case
is a kid trying to bully me and assert what eh believes are his rights
(mostly based on guilt trips) in a somewhat funny way to me
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Any cases of how some of you have handled these kinds of problems?
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I am also worried about the legal implications that impending
problems might have. When does a teenager become and adult?
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Thanks
lbrtchx