*Membership spots not really limited!
I actually have a solution for that. It's pure fan fic, but whatever. Hitler was into the occult, right? Between Vandal Savage, Wotan, and Mordru, you've got three villains that could fit a JSA vs. Occult Nazi themes. So part of that occult Nazi story involves searching for an artifact that grants long-lasting life and that affects the JSAers granting them said long-lasting life.
With some, the magic is already there. Alan Scott was shown as being brought back to life so why he aged at all... yet was rejuvenated in the old GLCQ... Jay Garrick and other super-powered types could just not age or age very fast being that vital due to their powers. Dr Fate and The Spectre are immortal.