It won't all be 18 year olds. There are a couple of students in their thirties on my course, and more who started in their twenties. We often invited one of the ladies in her thirties to lunch with us, and she came to my best friend's 21st birthday party.
However, university isn't like school, and you don't have defined "lunches". We often walked into the city centre for lunch when we had an hour's break at around the right time, but sometimes you will only have lectures in the mornings/afternoons anyway.
Once you get a personal tutor, hopefully they will be able to signpost you towards the support you need for your OCD/anxiety. You might also be entitled to some support from DSA, which the disability services should be able to advise you about. You might also be allowed to sit exams in a smaller room, and other reasonable adjustments.
No-one will think this is weird. Universities are generally quite good at supporting additional needs, and plenty of people will have assistance and special exam arrangements due to all sorts of needs.
University isn't like school, and there isn't the same level of cliquishness. You won't get people making fun of you in lectures- people are there to work and will usually be concentrating (or at least quiet). Bullying/meaness does happen in universities, but in my experience it's mostly confined to halls.
Don't forget all the 18 year olds will be feeling exactly the same, with the added worries of leaving home for the first time. I bet you'll end up really enjoying it and having lots of fun!