Thanks for all your replies.
Re the age specific comment, I'm not suggesting that people shouldn't wear whatever they want, but I know I would personally feel ridiculous wearing the cropped tops and distressed jeans that my 12yo daughter wears or the flowery blouses and sensible trousers that my 70yo mother wears. Some styles do lend themselves better to a particular age group.
Delighted to hear that nobody will care, as some of you have said this will be a very welcome change after years of working in an office! The majority of 22/23 year olds I know are recent graduates that have gone to work in offices and they tend to be be very fashion conscious with perfect hair and make up. Good to know that their peers who are still in education are still scruffy! 
So, if I base my wardrobe around what I wear at weekends:
Skinny jeans
Vest tops
Check shirts
Breton tops
Plain long sleeved t-shirts
Converse
New Balance trainers
Chelsea boots
Parka over the top when it gets cold
Bobble hat and scarf when it gets even colder
Rucksack for carting all my books around
I won't look out of place and/or nobody will care anyway?
I'll struggle with tying my hair up as I've got a pixie cut. Not wearing much make up won't be a struggle, I only really wear tinted moisturiser, mascara and lip gloss and tidy up my eyebrows.
This really is so much easier than I expected! 