Whenever you want to. We did it somewhere around 12 to 13 months b/c DS was not happy at all in his high chair. We switched to the booster and he was so much better at meal times.
DD was happy in a high chair right up until she turned 2. Then one day she was done. We never used a booster. DD refused. She now either stands in a chair or sits on her knees. I would make they switch whenever you feel your LO is ready.
I've never even thought about this. We have the FP Space Saver though, so it's kind of like a booster anyway. Now that he's pretty good about eating with a plate, I guess we'll take the tray away when he proves to us he's not going to tip himself over by pushing on the table with his feet.
At restaurants, we often use a booster for DS. He just does better in it. At home, he uses a high chair or sits on my lap. With DD, we moved her to a little table for a kid, since DH and I ate on the sofa. Her little table was at the end of the sofa and it worked well (we have hardwoods, so that helped. I would not rec this for carpet)
We've been using the FP Rainforest booster seats for a long time (we never had highchairs but we've used these and the First Years feeding seats back when we needed something that reclined for bottles). We do still use the trays though; tried putting them at the table a month ago and they kept pushing off with their feet.
Re: When are you supposed to switch to a booster?