I can relate to this a great deal. I'm 23 now, and feel the same way, but it's gotten better since when I was 18. I don't even like telling people how old I am. It's embarrassing, in a way. I've always gotten along better with people much older than me. Even my partner (and father of my child) is almost 16 years older than me. More than anything, other people's age just isn't important to me. But I find myself struggling to remember that when I'm talking to people around my age, rather than talking to people that are much older than me. I just relate better to those that have been around for awhile, and I always have. I consider some of my parent's friends to be my friends. It's a bit of a condundrum, because I can't seem to make friends with people around my age, but when I become friendly with people older than me, I feel like I can't really be their friend because they're just going to think of me as less of a person just because I was in diapers when they were graduating high school. Still, most people I've gotten to know on some level are pretty cool about it. Just do your best to put yourself out there (hard, I know) and hopefully people will see past your age, if they know what's good for them.
I struggled with that feeling superior thing, too. It's awkward, because a part of me does feel superior to people my own age, and a part of me feels completely self-conscious for being so different.