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]

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "impressionable":
   accessible, adaptable, alive to, amenable, apt, aware, bendable,
   bending, bright, clever, cognizant, compliant, conscious, delicate,
   docile, ductile, educable, elastic, emotionable, extensible,
   extensile, fabricable, facile, fictile, flexible, flexile,
   flexuous, formable, formative, giving, impressible, impressive,
   influenceable, instructable, intelligent, like putty, limber,
   lissome, lithe, lithesome, malleable, moldable, motivated, movable,
   open, open-minded, passible, perceptive, persuadable, persuasible,
   pervious, plastic, pliable, pliant, quick, ready, receptive,
   responsive, ripe for instruction, schoolable, sensible, sensile,
   sensitive, sensitive to, sentient, sequacious, shapable, soft,
   softhearted, springy, suasible, submissive, suggestible, supple,
   susceptible, susceptive, swayable, sympathetic, teachable, tender,
   tenderhearted, thirsty for knowledge, tractable, tractile,
   trainable, warmhearted, weak, whippy, willing, willowy, yielding