Regarding eating cattails past their prime. I assume you mean they have turned to the brown, full of fluff, seed pod. If this is the case, you may want to try this. The Paiute eat cattail seeds. A friend of mine and I were scratching our heads about how they separated the seeds from so much fluff. Finally we just lit a pile of fluff which of course disappeared with a poof and continued adding a little at a time until what was left was some nice toasty seeds. Tasty but tiny.
In addition to edible cattail seeds, spring buds on underground stems and young shoots can be eaten raw or cooked like asparagus. Cores of the young leaves can be cooked in the same way. Cook female flowers (male flowers are yellow above and female flowers are green below on the same stalk) like corn on the cob and use ripe pollen as a flour substitute. Starchy cores of underground stems in winter can also be ground into flour.
Offered by Steve.
Common cattails shoots are very edible and the roots work well as a substitute for potatoes.
Offered by Woodie.