When I was 20 I lived at home again during my sandwich year at uni. I was working full time, on a slightly-above-NMW salary.
My parents sat me down at the table and went through the household bills, and we worked out a contribution for me to pay towards it, about £100 I think. On top of that, I had my own car to pay for and run, and I continued to buy all my own toiletries etc as I had done at uni.
On top of that, I was also expected to do 1 food shop per month, and cook once a week (dad cooked at least once too). The food shop was usually basics and staples, bread, milk, eggs, pasta, some sauces, chicken etc. The obvious stuff. And I still took my turn doing the dishwasher, hoover, bleaching the loos, changing my bed etc.
I think that was a fair compromise. I learned early about budgeting, meal planning, contributing to general household upkeep. Basically how to live with others, in the safety of my parents home.
I'd like to think I'll do the same with mine if ever they live with us when they're adults.