All my kids are in boosters right now (ages 13months to 21 months), except for my daughter (3.5). She has been sitting in a regular chair since she was 2-2.5. I'm actually considering bringing my picnic table in from outside and setting it up for my daughter and the 21 months olds to see if I can free up some room in my kitchen. If they can sit properly and eat their snacks and lunch there it will save me from having to lift everyone into their chairs.
My son who is 21 months, actually doesn't like to sit in his booster seat because he wants to be like his big sister...he is just too short right now to sit without one at the table.