I have tried my hand at it sporadically, and have ponied a 16.3 hand draft cross, and both our Canadian mares, on more than one occasion. My 15-year-old Percheron cross gelding (15.2) is a -great- ponying horse: very steady, solid, does not kick and can take some abuse from a youngster without getting too phased.
Yesterday I took out the 5 year old Canadian and she was very good! I put her in a rope halter and we did our one-mile private road/laneway. However, she leans quite heavily into my gelding, and while she wasn't out of control by any means, she pulled quite a bit (my arm was dead after).
The young colt and gelding I foresee being the opposite -- they will probably be on the slow side to begin with and lag behind.
Does anyone have basic steps they take to teach a young horse to pony properly and well? How young would you start ponying? Any tips for me while riding? I find I twist my upper body when I'm trying to fight with either a dragging or a pulling horse...