Re: How can you tell when asparagus is too old?
I’ve never had an asparagus bed older than 17 years, but my understanding is that the crowns will literally die. If your bed was past its prime, I think that you would see bare patches in the bed; areas where it looks like asparagus should or used to come up, but is no longer appearing.
As far as toughness, I think there are several things that may cause this. A. Asparagus should be eaten shortly after it is picked, it becomes tougher the longer it sits in your fridge. B. Asparagus growing (slower) in cold weather will be tougher and therefore should be picked shorter than asparagus growing (faster) in warm weather. C. Once the heads begin to open up the asparagus quickly thoughens; it should be picked when the spears are still tight. D. Newer asparagus cultivars claim to be more tender than old ones; perhaps yours is not as tender as the asparagus you are used to eating.
If you feel that your bed needs rejuvenating, I would suggest that you top dress heavily and skip a year of harvesting. It would also be a good idea to get a soil test to see if the pH needs adjusting. This factsheet from the New Hampshire cooperative extension contains information on the appropriate fetilizer ratios: http://extension.unh.edu/resources/representation/Resource000603_Rep625.pdf