Have a meeting with your boss and the person who's treating you this way. Tell them that you'd like to fix whatever happened. Sometimes, we aren't aware of it because we are never told, just complained about.
I can tell you now, the stress that their behavior towards you is causing you (or will cause you if it continues) is not worth it. I worked in an environment where I was passively-aggressively bullied, if that makes sense. The stress I suffered, unknowingly at the time, caused complication in my last pregnancy.
You need to set it clear to them that you will not stand for their behavior towards you. They can say whatever they want to say behind your back but they will respect you as an individual and employee while in your presence and those of the customers. That's just poor workmanship on their part.
It could very well be that they feel threatened by you. You are the new blood in the office and they are older. They are afraid that you will replace them. Still, it doesn't give them an excuse to treat you this way.
Good luck. If things don't change after you talk to them, start seeking other employment and leave as soon as you can. When asked why you want to leave be honest with them. It'll probably open up your bosses eyes. Who knows how many they've treated this way before you.