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As per this link, the word 'kidnap' originated to denote nabbing away of a child. When and how did kidnap come to denote nabbing of adults? Just found a link to a 1650 book that mentions

Once more, though, this is a term that is used by older people towards younger ones. It is more used for teenagers and. I have three age groups that i want to distinguish in my research project. They are as follows: 18-45 years - i have called this group young adults 46-65 years - i have called this group adults 66