Quinn, a Democrat, signed the bill Wednesday at Adlai E. Stevenson High School in Lincolnshire, where faculty and students pushed the legislation, the Chicago Tribune reported.
Illinois joins at least 20 states that set younger age limit for primary voters, Quinn said, citing a national voting advocacy group.
Rep. Carol Sente, a Democrat and sponsor of the legislation, said in a statement the measure was "only fair" since 18-year-olds "are already voting for candidates in the general election."
The law takes effect Jan. 1.
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