As is stands Ds23 empties the dishwasher 3 days a week, I do 2 and Dh does 2. These days are on a rota. When Ds20 is back from uni (same as when eldest Ds was back from uni) we revert back to the original dishwasher emptying schedule where they do alternating days each including a Sunday or Monday for a weekend load and I do the Friday. Dh gets off the dishwasher schedule but this man shops, cooks and BBQs. We still have the rota so I just bring it out from the cupboard and stick it back on the fridge.
Bin emptying is done on set days, one child is responsible for the kitchen bin, one the recycling. On bin collection day they put their bin out and bring it back in. Again when home from uni, the person resumes this role.
I think that these the only things that change. They both do their own laundry although if they wear a white t shirt it goes into my laundry as I have a whites basket. They can both cook as both have/are away at uni. They both have clean and tidy bedrooms and they share a bathroom, again this is clean.
As your youngest is working at a lower age level, maybe the eldest can tandem cook with them as a sous chef as we called it in this house. I have a 3 year age gap so mine did this and still do this now. Work out what you think they would be capable of making, get older sibling involved if they get on.
Lunch is the easiest way to start, sandwiches, salads. This is good if you are working from home and you can have your lunch made for you. Clearing up after themselves too which your eldest should be doing as they are away at uni.
Lay out the expectations. Then everyone knows what is expected of them and their role.
I menu plan, some things are the same every week like Friday night curry, all of us can make this, jar of paste, frozen veg, frozen coriander, rice, poppadoms. Again this was something we all sat down to discuss, made a list of all the meals we like to eat under different headings, meat, fish, veggie etc. That way you have a list to pick from and they can pick things they can make.
Now your eldest may be working but so are you, they too must make some dinners, it cannot always be you because you are their Mum.