I did this :-)
I was sick during my A-levels so had to do them over three years. Then first time I tried to go to uni kept getting hospitalised...when I went back and did my proper first year I was 20 (21 in early Nov) had a great time and have made great friends that I still have seven years later! I'm now doing a masters and have started to make friends with 22 year-olds (I'm five-six years older) but then there's a woman in my course in her 40s/50s and her input and life experience is so valuable (we were discussing eating behaviour weight gain in pregnancy and post-partum - she was the only one of us who was really knowledgable about this).
Honestly, if you are a sociable, friendly person you'll make friends and fit in! I made friends despite not being able to go out most of the time (and Bristol uni is big on clubbing/ night life - most of the friends I made were party girls, despite not being able to do that, because that's who I would have been if I hadn't had the health stuff but they were happy to have more chilled nights with me when they needed to rest)! Also, one of my friends I made was the same age as me! She took a gap year then didn't like her course at the first uni she picked...two years older isn't that uncommon and there will be loads of people who are just one year younger (gap year students)!