I’m not 18, but will be by election day. Can I register?
In some states, YES. In all but a few states you are entitled to register to vote if you will be 18 by the upcoming election day. In the following states you can only register if your 18th birthday is within a certain number of days/months: AK – 90 days; GA – 6 months; IA – 6 months; MO – 6 months; TX – 2 months. If you qualify under these rules, go ahead and register now.
For more information (and states where you can even register at 16!), check out this page from the NCSL.