C.R.S. § 1-2-101

(1) Every person who is eighteen years of age or older on the date of the next election and who has the following qualifications is entitled to register to vote at all elections: (a) The person is a citizen of the United States; and (b) The person has resided in this state twenty-two days immediately prior to the election at which the person intends to vote. (2)(a)(I) Notwithstanding subsection (1) of this section, upon satisfactory proof of age, every person who is otherwise qualified to register and is sixteen years of age or older but will not have reached eighteen years of age by the date of the next election may preregister and update his or her preregistered information by any means authorized in this article for persons eighteen years of age or older. Upon reaching eighteen years of age, the person is automatically registered. (II) Repealed. (b) The registration requirements of section 1-2-201 apply to a person preregistering to vote under this subsection (2). (c) A person preregistered under this subsection (2) who is seventeen years of age on the date of a primary election or presidential primary election and who will be eighteen years of age on the date of the next general election is entitled to vote in the primary election or presidential primary election. (3) Repealed.