Maybe you need to read the entire thing? The constitution originally left voting rights up to the states to sort out individually. It was the 14th amendment adopted in 1868 that universally set the male, 21, born or naturalized citizens standard. Technically, that doesn't stop the states from still doing what they want though. They could allow non-citizens to vote if they really wanted to. It is a right and there is nothing in the constitution that prohibits it.
The full list of protections from Wikipedia:
These extensions state that voting rights cannot be denied or abridged based on the following:
Birth - "All persons born or naturalized" "are citizens" of the United States and the U.S. state where they reside (14th Amendment, 1868)
"Race, color, or previous condition of servitude" - (15th Amendment, 1870)
"On account of sex" - (19th Amendment, 1920)
In Washington, D.C., presidential elections (23rd Amendment, 1961)
(For federal elections) "By reason of failure to pay any poll tax or other tax" - (24th Amendment, 1964)
(For state elections) Taxes - (14th Amendment; Harper v. Virginia Board of Elections, 383 U.S. 663 (1966))
"Who are eighteen years of age or older, to vote, shall not be denied or abridged by the United States or by any state on account of age" (26th Amendment, 1971).
Requirement that a person reside in a jurisdiction for an extended period of time (14th Amendment; Dunn v. Blumstein, 405 U.S. 330 (1972))