20 years old doesn't mean much, especially since unless he is registered, there is no way to tell if he is 20 or 25... What is far more important is his health and your recognition that things can change quickly if he is not properly cared for. Invest in a FULL vet check. You need to know if there will be things you need to deal with before you are ready. Generally speaking though, 20 years old is not too old, especially for a child. It's actually better for a child to have the older horse. However, keep in mind that if the child is 8 now, when (s)he hits the teenage years, the horse may not be able to keep up.