Age is only a number as you're as young as you feel. It sounds like you've already sabotaged your trip even before going here. They all know youre older but don't care about that They are inviting you as a friend not a chaperon here so go and have fun and things will work out.
My gf from my college is going on a 3 days trip with her classmates. She wants me to come. but all are my juniors and I talk to none of them. I don't know what to do. Being a senior I feel I'll feel awkward among them as I dont know them too?
In the adult world we socialize with everyone not just people we know. Being a senior means nothing about who you are was a person or who they are as people. You can learn a lot from others and befriend them and find them interesting, who cares if they are seniors, pink purple high schoolers, smart, religious political, they are people fellow humans.
If you can't be comfortable and step out of you little box, then you need to know that in a short time you are not a senior but with all the other adults and you had better be prepared for surviving with fewer labels of comfort.