When we went last Sept. our youngest was 7 months, and a premie. He just started sitting up 2 days before our trip. I was nervous about that b/c when I called, I was told there were no carrier slings (those wooden or plastic contraptions that hold a baby carrier/carseat). Turning a highchair upside down is dangerous, exteremly dangerous. There are usually warnings on the chair and many restruants won't allow it. I used to wait tables, then manage, then consult before mommy-hood in restruants, so I know what I'm talking about. Some highchairs are made to fit, most if not all, infant carriers. I wish all restraunts would switch to those if no baby slings.
Anyhow, we didn't have a problem getting highchairs. Call ahead to the places you want to eat if it is a question