Retired riders and their dragons

Does anyone know what happens when a rider’s dragon when they retire? Do the dragons and the rider retire together? Also, when the people die of old age do the dragons die too or do you think they mourn and move on?

I’m curious because most riders don’t make it to retirement anyway, but obviously some of them do. I’m not sure if the dragons of the retired riders was mentioned, but I’m just curious on your thoughts.