Note at 20 you aren't actually even considered a mature student formally (unis consider students "mature" students if they're over 21), and there's virtually no difference between you and any other school leaver as well. You're essentially a school leaver plus 2 gap years (or restarted A-levels/retook A-levels plus gap year) - not uncommon, and people starting after 1 gap year are very common.
Remember that uni (and more to the point, everything else in your life from now until you die) is not age segregated in the way that school was. While I appreciate the anxiety over such matters because up until now that's all you've known in life, it's essentially immaterial going forward. In your degree, in the modules you take there may be students of a variety of ages - not just due to mature students, but people retaking modules/the year, students from other courses taking your modules as an option, people taking the same optional modules as you but in a later year of their degree etc.
There is nowhere near the level of homogeneity in age demographics per "course" or cohort at uni as there is in school.