all of this depends on what you want to do with the shetland . Do you want to ride ? If so I would have a vet check , and make sure she is in good shape , check for lameness, check her back , and ask the VET if she can hold the weight. If you want to show , ask the owner what she has accomplished in the past, is she trained ? can she clip , tie , load , bathe , stand for the ferrier , lunge etc. At that age I would hope that she has had some training. If you just want a shetland to introduce your child to horse care , and disapline , 20 years old is perfect, even better if she is calm and well trained. Later when you learn more about the breed , and know which direction you want to go in it would be nice to purchase a friend for her. Maybe something a little younger . If it were my child I would start with a very sweet , calm little mare, that is completely trained in ground manners, so this is a positive expierence for your child. Also it is important to ask the previous owner for all medical info on her. Read the topics in this forum and you will learn a lot about this wonderful breed. good luck