Answer:
1. I would ask to speak to a higher member of staff or take my business elsewhere if they still refuse to serve us.
2. The staff member could have a preconcieved opinion that young people are rude, disrespectful, loud and irresponsible. This opinion would most likely contrast to an older group of people.
3. No. Being stereotyped due to your age and refused service based on that is both unnecessary and unprofessional. It would be different if there was a disruption caused by me and my friends but nobody can forsee that on someone's entrance to a cafe.