The Collaborative International Dictionary of English v.0.48:

Impressionable \Im*pres"sion*a*ble\, a. [Cf. F.
   impressionnable.]
   Liable or subject to impression; capable of being molded;
   susceptible; impressible; as, a bad influence on
   impressionable youths.
   [1913 Webster]

         He was too impressionable; he had too much of the
         temperament of genius.                   --Motley.
   [1913 Webster]

         A pretty face and an impressionable disposition. --T.
                                                  Hook.
   [1913 Webster]

WordNet (r) 3.0 (2006):

impressionable
    adj 1: easily impressed or influenced; "an impressionable
           youngster"; "an impressionable age"; "a waxy mind" [syn:
           impressionable, waxy, impressible] [ant:
           unimpressionable]