My six year old daughter has been flying with me in a PA28 since she was 18 months old. I still use her car booster seat and she now sits up front with me. A word of warning. There was one occaision when just as I flared to land, she leant forward and grabbed the yoke as it approached the fully rearward extent of travel to "help Daddy land the plane". I'd taken the precaution of moving her seat back but she still had just enough reach to grab on! The resultant balloon was probably no worse than some of my other landings but a bit of a shock at the time!