Imagine my embarassment once when I was teaching a community photography class, and wanted some on-site experience at the (then) world's biggest shopping mall in Schaumburg Illinois. We were pointedly asked to leave. Something about the window decorations being copyrighted... Save me from lawyers! tasquah has the right idea; pay the model up-front, get a signed release, and photograph the age on their drivers' license. Then, photograph whatever you can talk then into.