My 6 month old (but large) son has a Baby Bjorn Active chair, which is supposed to be suitable to age 2. He keeps leaning forward/sideways, and it loks very unsafe. However surely they're designed to stop that happening?
DD managed to throw herself out once at about 6 months so I put the chair away. TBH I know that the instructions say that you can use the chairs up to 9 kgs or further but I've never been able to get more that 6 months use out of ours before I've felt it has become no longer suitable for my babies.