You can still show in the age classes, even if you are a champion, as they are, obviously, only age restricted. Then you have to move into Limit and Open.
As others have said Junior and Yearling ticket winners can still be shown in those classes until they are out of the age. After that the lowest class a cc winner can be shown in is Limit which applies until they have won thier 3rd ticket or they have won 7 limit or open classes at Champ shows where CCs are on offer.