I am 26 so I'm closer in age to 18 than my kids are. I used to sit and eat with my parents some evenings, but not every day - at 18 you're an adult and tend to be going out to see mates, boyfriends etc in the evening fairly often! In fact I was only living at home for 4 months after my 18th birthday because after that I was off to uni already and by that point they're definitely not going to be sitting at the table with you eating dinner anymore!
As long as you keep good communication so you don't end up making dinners that go to waste, I don't think it matters whether adult children eat with their parents or not.
Unless your question is really "do your 18 year old kids eat at the table or in their room?" in which case that's a totally different question - if everyone is eating at home at the same time then I would expect everyone to sit and eat together. If they come home late and make a pizza for themselves way after you've eaten, that's different again.