Anon,
If you want to go back to university, then go for it!
You have hopefully had time to think about what you'd like to study and can reflect on what was good about your last university experience and what wasn't great : (
Do your research. Make sure you have a good understanding of how the course is taught and the assessments and how this may differ at different universities. Think about what's important to you about a university and make sure that the course is something you can see your see yourself studying for three or four years.
Remember that people do gap years and some people have late birthdays, so not everyone will be 18. There may also be people who have studied before but have decided to change careers. Depending on your course or the university you attend, you might be more or less likely to find students who are older, but just to reassure you again, 21 is still young!
All the best,
Oluwatosin – 2nd year student – University of Huddersfield