Keeping a horse active & happy is very important. But just take in consideration that she may not be able to complete her job as easily as she used to, even if she still really enjoys it. If you start to notice that she is uncomfortable or uneven at all, that would be a good time to consider taking it down a few levels. That doesn't necessarily mean retirement immediately, but jumping is a very hard and impacting exercise, especially for older horses. :)
When it gets to the point your having to force her to move or you feel it's not worth forcing her to move or if she is in pain due to aging. We have a 26 y.o made at our barn who still runs barrels and fast. But there's also a 19 y.o gelding that is just used for rising lessons because he has bad arthritis. It just depends on the horse.