Voters under age 2o and especially college age "children" are not sufficiently in possession of knowledge in terms of candidate position or party platforms. Most children that age are simply tools of a third party, i.e. teacher, professor, etc and not in possession of facts or information sophisticated to make an informed ecision.
Most people under this age are not tax payers, they are not vested in retirement programs, lacking history of the world for defense issues and simply unprepared to select a candidate. Most children this age will select a candidate that appeals to their sense of emotion.
This is not to say that many adults over 21 are at a greater informational advantage. Selecting voters that are swayed by emotion is not contributing to the quality of candidates.