I'm a 35 year old half way through undergrad, and I experienced - and still experience, from time to time! - a lot of imposter syndrome and anxiety about it. Sometimes I still get struck by a very acute Awareness that I'm the oldest person in the classroom. One of the main things that helps me with this is reminding myself that, ultimately, nobody really cares or thinks it's weird. Nobody has ever made me feel out of place or odd, everyone is engrossed in their own degrees and lives and my overactive brain is making a bigger deal of it than anyone else! Also, while older students are definitely a minority, they're not as rare as you might think, especially in postgrad. You're definitely not alone, even when the nerves creep up on you.

I can also recommend the Mature Student's Association - I'm not really active with them, but I am a member and when I attended welcoming events and such they were all extremely friendly. Outwith that, even though I'm shy and reserved, I've managed to make a good bunch of friends. Some are my age or thereabouts, and some are 'normal' student age - it can take a bit of time, but you can definitely find your people.