Teenagers and young adults are a tough audience to reach. Could an anti-littering message that capitalises on their desire not to be seen as children be effective?
This poster is a quick mock up I put together showing how this could work. While I am not 100% happy with the text (it’s bit too in your face for my taste), I think it clearly gets across that littering is childish. When dealing with an age group that tends to rely on image and fitting in to get by, creating this negative association with littering may get the message across.