If they live at home, obviously.
DD is 24 and I cook all her meals - she likes different food to the stuff DH and I eat and we eat dinner later than she does so I usually cook her meal separately in the evening. She doesn't cook herself but makes herself a sandwich for lunch sometimes.
She pays board so I always considered this to be fair enough, until I saw someone on here mention that they don't cook for their adult DC.
Do you cook for yours? Do they cook for you? fantasises about someone else making my dinner for a change