A 17 year old can cast a vote in a primary election if they will be 18 by the time of the general election.
Active and challenged voters both appear on the voter checklist, and both are eligible to vote. A voter's status can be challenged by the local Board of Civil Authority when, for example, that person fails to vote over a period of time, and may eventually be removed from the checklist if they do not respond to the letter of challenge.
Most voter records contain a birth year, which allows us to estimate the mean approximate age of voters in any given district. This data has improved over time: currently 84.5% of active voters have known birth years and 98.2% of challenged voters do.