Out at 18 - really?

Why do Americans throw their children out on the street or treat them as renters at age 18?
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A friend of mine's wife kicked their 18-year-old son out of the house just after he graduated from high school. He was allowed to take only his clothing and an old car that stopped running shortly after he left. He wound up living in a park and begging for food and money while trying to find a job with no success. After 3 months in the park, he was found dead one morning, having been murdered. My (former) friend and his wife decided it was his own fault for not getting a job.
Out at 18 - really?
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