When I moved into my first flat, I didn't have much money, so it was more a case of "how can I eat as well as I can, for the money I have to spend", whereas I know quite a few folk who have a comfortable amount of money, and spend vastly differently from when I started out.
When students move into halls and start self catering for the first time, you'll find a lot of parents set them up with a first shop, including all the things you don't buy very often, otherwise the first couple of times you shop gets very front loaded. eg a bottle of washing up liquid or a box of washing powder will last you for weeks, but you can't wait until the 8th week to buy it - you need it as you move in. Ditto a box of teabags or a bottle or ketchup, spices etc etc.
Once you have all those things, then it can settle down, but it depends which shop you shop in; if you are a 'snacker'; if you like fresh fruit; if you are snobby about brands; if you have strong views about 'organic products/ ethically produced products; if you have freezer space ; how good your cooking skills are ; how much time you have, etc. etc
All that said, on a weekly basis, my 19 yr old spends up to £20.