You can buy the blow up thingy too at ikea ($9 i think) for when they are littler and just starting out, it fits in the ikea chair behind their backs.

I put my girls in a bumbo at 6 months when it was feed time. They were not sitting up fully by 6 months, and besides i thought they looked so tiny and frail in this huge looking high chair. I found the bumbo easier when bub first trying solids.

HTH