I graduated at 22 and didn't have trouble making friends at work. (Although if there were cliques, I wasn't typically included.) I was in my 50s when I started my last job, and the other nurses were mostly of the same vintage. I really enjoyed working with them. Then they all started retiring.
They were replaced by a much younger set. Really great people. Top notch conscientious nurses, fun and friendly. However, they seemed to gravitate to one another much more than they did to me. No one ever actually snubbed me, and always treated me with friendliness. But they clearly related better to one another. I started to feel excluded though I am positive no one meant for me to feel that way.
So, yes, I think it's an age thing. I always thought that becoming a nurse later in life would bring its own special challenges. I always believed that if I hadn't become a nurse by my late thirties or forties, I probably wouldn't have bothered. Not telling anyone else they shouldn't have; we all have our own lives to live.
I think for those who are currently in that boat, enjoy that you are doing what you want to do, find meaning in it and enjoy your paycheque. Look elsewhere for camaraderie. And do not tolerate mean girl behaviour. You have the wisdom and maturity to nip that crap in the bud.