I did Intensive outpatient rehab for about a month and it was a mix of all ages. I was nervous like you being in my early twenties and a female. You really see that addiction does not discriminate and everyone is in the same boat. It was a great place to get clean and gain support so that I could put my life back together. There were mixed groups and then there would be all female group sessions, usually alternating throughout the day. You'll probably feel a little more comfortable after actually seeing what it's like.
Also you really learn a lot about yourself and how strong you really can be. My advice aligns with Raider above^ in that you just give it all you got! it's a gift to be given a second chance when you're that young. I thought people would judge me too but I found that everyone understood that I took this seriously. Best of luck, keep an open mind, and don't over think it. Everyone's usually there to work on themselves.