I was older and kept to myself. I would help people without being asked, but didn't do the study group thing. Turns out it was better for me since I found the study groups became more of a social than study thing. I did however hang out with the students who were closer to me in age. We would get together in clinicals and during breaks to quick crams and pass it on.
Regarding the others not wanting anything to do with you 4 months in. I'm not saying their behavior is right, but did you try and connect before your friend left? Your tone does sound a little angry. You said you wanted friends to make the class fun and that a classmate said you didn't work hard enough. What gave them that perception?
My class was very diverse in age and there were a mother and daughter (your age) in class. The daughter originally tried to be part of another group her own age, but came back to Mom and us old folks because her perception of her peers were that they didn't take it serious enough for her. She said her study group had members late or never show and not much studying being done. The daughter needed to prove to group members (other than mom) that she worked hard.
I'm not trying to give you a hard time, but becoming a group requires acceptance and respect. Both of which need to be earned and aren't automatic. Show them you're serious with your actions. Once you're accepted (and it won't take too long) you can show them your fun side.
Best of luck to you.