Ritzville, Washington, skull too old to be Sofia Juarez or Cody Haynes

I have sometimes heard that carbon-14 dating is not completely reliable, but still all I can think is, "Thank goodness!" Would age also explain why they could not obtain DNA?
Story from http://seattletimes.nwsource.com/html/localnews/2003996829_webbones06.html:

Tuesday, November 6, 2007 - Page updated at 08:26 AM
Skull found in field
near Ritzville is 600 to 770 years old
The Associated Press
RITZVILLE —
Pieces of a small skull that were found in a field south of town along U.S.
Highway 395 last year have been found to be 600 to 770 years old, Adams County
sheriff Douglas Barger says.
The results of FBI carbon dating ruled out any
link to two missing children in the region, Barger said Monday.
Three bone
fragments that were found on Oct. 12, 2006, after a backfire from a car started
a fire in a field south of Ritzville were quickly identified as pieces of the
skull of a child, but DNA testing was inconclusive.
No other bones or human
remains were found in an ensuing search.
Authorities hoped the remains might
shed some light on the disappearance of 11-year-old Cody Haynes of Kittitas in
September 2004 or 5-year-old Sofia Juarez of Kennewick in February
2003.
Richard Haynes, the missing boy's father, was under scrutiny from
authorities in Kittitas County. He reportedly told investigators he took a
meandering 250-mile drive — including the stretch of U.S. 395 where the bones
were found — in search of auto parts the night his son vanished. He said he
believed his son was a runaway.
The little girl disappeared after being given
$1 by her mother to buy candy at a convenience store.
The carbon-14 testing
made it clear that the skull pieces were from an American Indian, so steps are
being taken under federal law to return the bone fragments to tribal
authorities, Barger said.
The Washington state Department of Archaeology and
Historic Preservation also has been contacted, Undersheriff John W. Hunt said.

If you know anything about the disappearance of either of these children, please call the National Center for Missing and Exploited Children at 1-800-THE-LOST (1-800-843-5678). For Cody, you can also call the Kittitas police at (509) 925-8534. For Sofia, you can also call the Kennewick police at either (509) 585-4208 or (509) 628-0333.

Cody's poster: http://www.missingkids.com/missingkids/servlet/PubCaseSearchServlet?act=viewPoster&caseNum=996768&orgPrefix=NCMC&searchLang=en_US
Cody's website: http://www.codyhaynes.org

Sofia's poster: http://www.missingkids.com/missingkids/servlet/PubCaseSearchServlet?act=viewPoster&caseNum=954908&orgPrefix=NCMC&searchLang=en_US
Sofia's website: http://www.geocities.com/thekarmaclub/sofia.html