As to GL things being temporary...true, except for things that physically change the actual world around it....like if I knock down a tower, the tower stays knocked down. If I use the ring to mix mortar, and simultaneously lift bricks and mortar to build a wall, then the wall I built will stay up, because the alteration was made with the power, but wasn't in and of itself using the power.

So if my enhancement physically changes the cells so that leaving them alone in the new, altered state, I could see de-aging etc.

Protection from disease, healing of chronic issues, etc. would likely have a prolonging effect even without intervention....now, that might mean you're more likely to hit 110 years old, or even get into the 120's (I think 120 is about the max any human has ever lived), but I don't think it would let you live to 1000 or anything, or have no degradation without expressly de-aging yourself, which may well be within the capabilities of the rings, at least classically....GLs don't have a history of being very creative with the rings, after all....Hal was the first to use red shifting to fight yellow, for instance....

So, being creative with the ring, it may not be as bad as some other items, but there are still limitations that wouldn't be present with the power of shazam...