The Collaborative International Dictionary of English v.0.48:

Whiten \Whit"en\, v. i. [imp. & p. p. Whitened; p. pr. & vb.
   n. Whitening.] [OE. whitenen; cf. Icel. hv[imac]tna.]
   To grow white; to turn or become white or whiter; as, the
   hair whitens with age; the sea whitens with foam; the trees
   in spring whiten with blossoms.
   [1913 Webster]

WordNet (r) 3.0 (2006):

whitened
    adj 1: (of hair) having lost its color; "the white hairs of old
           age" [syn: white, whitened]