I don't think there is too old. Why don't you go to an agility competition day and have a look. There's a lady at my club - I think she's well over 60, doing agility with her welsh springer spaniel. And there are several ladies including a few over 70 still playing hockey.
The best agility handlers - don't actually run much, they stand in strategic places on the course and point and call and their dog does the appropriate obstacle in the right direction and it's truly spectacular to watch. Same deal with herding comps. Handler shouldn't be running much. But learning herding sometimes involves being crash tackled by stroppy sheep. Duck herding might be easier.