Someone who is impressionable is easily influenced. An impressionable person can be greatly changed by his or her experiences — not always in a good way.
When someone makes an impression on you, you remember them and are influenced by them. Likewise, someone impressionable is very susceptible to being influenced, almost like a sponge that soaks up ideas. This word almost always applies to kids who are often referred to as “young and impressionable." If you call someone impressionable, it usually implies that they could be easily led astray.