Where is the line that an older man shouldnt cross with a young girl ?

There is no definite line between behavior that is socially acceptable and not so and it is hard to tell when it's being crossed. A large portion of it depends on the persons up bringing and beliefs. For example touching someones leg could be normal to some and completely over the line for others. The really important thing is how comfortable you feel. If anything the person does makes you feel uncomfortable you should confront them. If their intentions weren't sexual they should have no problem apologizing and refraining from repeating the behavior. If their intention were sexual then you are aware and can get out before anything more happens.