In their recent article, The Center for Information & Research on Civic Learning and Engagement (CIRCLE) finds that "youth voter registration is up from 2016, but lagging for youngest eligible voters."
CIRCLE's latest look at voter registration data five weeks before the election shows that "in 21 of 27 states with recently updated data, voter registration for youth ages 18-24 is already higher than in November 2016." The data also shows that "in most states, voter registration among young people ages 18-24 is also higher than in 2018. In 14 of the states for which we have data from September, voter registration for that age group was higher in 2018 than in 2016, AND is higher now than in both years." And, "in more than half (16 of 27) of the states we looked at with updated data, registration among the youngest eligible voters is still below November 2016 levels."