Don't worry, there will be plenty of people in your year who are same age or even older. I started uni at 20 too and no one cared. No one actually even asked how old I was until I was like 4 months into the course and even then it was just for teasing a guy who was younger
I'm the youngest in my uni flat at 18 going on 19. Everyone else retook or had gap years. On my course we have people in their 40s starting their degrees. No one cares about age. We all just want to have a good time whilst desperately passing our exams and coursework Believe me, 20 is not old
I'll be starting uni in September at 34, and honestly I'd find someone one year older than me super weird. Like woh! Hang on, how do I even talk to this person? They've got a single years more life experience than me, will they even know how to use a computer being such a dinosaur?