The difference between having my parents with me and without while I was looking at purchasing a car was quite huge. I walked in with my friends because we were both interested in a car, but no one asked us if we needed help for the first 20 min we were there! Yes, we look young and may not be able to purchase it for ourselves but I don't think people in their early 20s would walk into a car dealership just for fun on a Saturday afternoon. A couple salesperson looked at us but didn't come towards us at all. After 20 min, finally a secretary asked if we needed help and introduced us to a salesperson. We were able to test drive a couple models and got a quote about the cars and asked for other details but they weren't like willing to answer.
The next time we went in with our parents, they serviced us right away and explained the car and price more thoroughly than when we were there by ourselves. As a salesperson, even if it doesn't look like we are gonna purchase a car, it's good etiquette to at least say "Hi, is there anything I can help you with today?" That takes about two seconds of your job.