Hi,
You won't be classified as a mature student by the university, as you have to be over 21 when STARTING the course. If you start another course after completing this one then you will be classfied as a mature student for that course, but for the purposes of the course you have currently applied for, for the duration of the course you will not be classified as a mature student.
You are indeed correct, after 4.30pm you don't need to pay or have a permit to park at Kedleston Road, however you aren't guaranteed a parking space.
Hope this helps,
Stacey