Illinois election officials say nearly 8,000 17-year olds have registered to vote in the March primary.  Under a new law, teenagers who will be 18 by the time of the November election may cast a ballot in the spring primary.  Rupert Borgsmiller is director of the Illinois State Board of Elections.  He said Friday that 7,867 registrations came in from eligible 17-year olds.  Registration for the March 18^th primary closed last Tuesday.  But teenagers may still indicate their preferences through grace-period voting, which runs through March 15^th.  Lawmakers who pushed the initiative said it would expand participation in the electoral process.  Illinois is the 20^th state to allow 17-year olds to vote in primaries.