Link to National Conference of State Legislatures (NCSL) list of states that allow citizens who are under age 18 to pre-register to vote
This procedure allows individuals who are younger than 18 years of age to register to vote, so they are eligible to cast a ballot when they reach age 18
Direct Link: http://www.ncsl.org/research/elections-and-campaigns/preregistration-for-young-voters.aspx