I started my undergraduate degree at 20 when everyone else was 17 or 18. No one knew I was slightly older unless I mentioned it. I lived in a nearby town with my partner in a house that he owned, so I really didn't see other students outside of class. While I was perfectly happy with my living situation (it would have been weird and a bad idea to move out of my partners house to another town when we were perfectly happy living together), it did mean that I was pretty isolated. I didn't really have any friends at uni until the last couple of years (BAs are 4 years in my country). My partner had to move so this year I've lived in private housing near the university and I've made good friends.
I think there will be some situations where it is a little weird. I don't know about you, but by the time I was 20 I had already finished my partying days so when lots of students in my year were going out binge drinking on weekends (or weeknights) I was not at all interested. I feel like there's often a certain difference in the maturity level of 18 year olds and twenty-somethings. Having said that, I'm sure you will meet lots of younger students with whom you'll get along. My uni is tiny (2500 students) and I've found lots of people with similar interests who are mature and serious about their degrees.
I would suggest getting involved as much as possible in social activities or else it might be a lonely few years. You will definitely find lots of people to hang out with, and I'm sure there will be several other older students. Taking a couple years off first was the best decision I ever made. In Canada you don't even have to pick your major until second year, so tonnes of students start uni with absolutely no idea what they want to take and end up wasting a lot of time and money. By taking a couple years off I knew exactly what I wanted to do and was able to get through quickly without much trouble. You'll have life experience that younger students don't so sometimes it might be a little tough to relate and tempting to "share your wisdom"... but I would say don't make a big deal out of your age difference and you'll have a great time, when you meet the right friends it will hardly ever occur to you that they're a few years younger.