When I started working full time (many years ago!) I was earning the grand total of £7,500 a year. I was conscious, however, that money had always been really tight growing up. My parents never bought each other birthday or Christmas presents, for instance, because they'd spend everything they had on making my Christmas special. We never went on holiday because we couldn't afford it and every penny was accounted for and budgeted for each month.
My parents never suggested or expected me to contribute to the running costs of the house, but as a responsible 18 year old I thought it was my duty to do so, so I offered and they accepted. Why should I have continued living off their extremely tight budget when I was earning my own money?
As others have said, a grown up child living at home will cost the household more money - child benefits disappear, council tax may suddenly go up without the 25% single person discount applying any more. That child is well within their rights to leave home and would have to start paying their own bills and food, etc. So why should they not learn some financial responsibility while living at home?
There is a big difference between the households that can and can't afford to subsidise their adult children living at home.
Personally I would expect all adult children to contribute to living at home. It's what the parent has to do with it that will differ. Some will need that money to pay towards the bills, and others will be able to put it aside to give back to them.
I like PP's suggestion of telling them that you'll save half for them, but saving all of it if you can.
It does help them learn to stand on their own two feet, IMHO.