I started putting DD1 in a high chair at 4 months - but not to eat. I just put her in there while we ate. I didn't start feeding her solids until 6 months.
I put DS in a high chair around 6 months old. He was sitting up on his own around 4 months, so he could have easily sit in his high chair without a problem at that point.
We have the FP Space Saver (used it for DD as well); it reclines so we use it now so that DS is in the kitchen when we are. For feeding, we started feeding DD at 5 months so used it then for that.
Re: Feeding