Vittorio sums it up
.. a date of birth gathered during some questionnaire or a date of birth certified by an authority carry different business weight ..
As yet, AFAIK, there is no standardized syntax by which these differences might be described - something equivalent to SAML's Authentication Context but for attribute statements and not authentication statements (pause here for somebody to mutter to themselves 'Well identifiers are just attributes after all').
Beyond syntax, what of a framework for attribute assurance? OMB m04-04/NIST 800-63 explicitly state that attribute assurance is out of scope for that work.