Kal-El, Clark Kent, The Man of Steel, Superman. The Red and Blue Boyscout from another world with abilities that far surpass the average human. Hell, it even surpasses most of the advanced humans. Bulletproof, the strength to lift school buses as if they were an empty cardboard box, faster than the bullet train, Flight, X-ray vision, Heat Vision, Ultraviolet Ray vision, squiggle vision, you name it. And with freeze breath, the Last Son of Krypton never needs a breath mint (unless he likes the cool taste of mint in his mouth, as well as other chemicals with his super-tasting ability that will probably put Daredevil to shame.

Of course he can do so much, but even someone who once held the whole planet on his shoulders at one point, has their weaknesses, none more prominent than the nasty green space rock, Kryptonite. For humans, its' effects do not show any immediate effects, but you can ask Lex Luthor how using Kyrptonite over a long term has affected him. For Supes and any Kryptonian, it makes them a bit weak in the knees to say the least. Of course, there have been various versions of Kryptonite, such as Red, Blue, Gold, and the infamous Pink Kryptonite, each with their own special effects on the premiere comic book superhero.

Kryptonite, like any other radioactive substance cannot transfer any of its radiation through Lead, which leads me to the question: why doesn't Superman just wear a lead suit? What made me think of this question in the first place was watching Superman: TAS, where Supes donned a grey containment suit lined with lead and blue gloves to deal with Metallo and his Kryptonite heart. I get it from the creator's standpoint in that his original suit is iconic and I can also get that Superman, being one of the most humble/'human' characters in the DC Universe (if you believe that), may want to have a weakness so that if someone were to take control of him and there was no other way to reverse the effect, the other heroes would need a way to stop him, but from another viewpoint, who is seriously going to get close to Superman to stab him with a Kryptonite blade or have nearly precognitive accuracy to hit him with a Kryptonite bullet or missile without his super--speeding off or hitting you and the weapon with Heat Vision?

Alternate Universes and retcons have been a prevalent way of 'restoring' characters from the golden days before they become golden guys and golden girls in their golden years.  We can look at Batman or Superman's first appearances and how they appear today, and although nothing too drastic has changed since the days of Action Comics #1, A new age will always reinvent these classic characters to fit in with that time. Granted, some characters I would not like to see as they were then as they are now (Black Lightning and Luke Cage with Afros for example), but  Yet, what if they remained in a certain time period and simply continued to grow old as new characters come in?
 
We see a glimpse of what happens when a hero gets old in Batman Beyond, which has Terry McGinnis taking on the role of Batman, while an elderly Bruce coaches him from the sidelines. While nature finally caught up with Bruce, it seems that for the Man of Steel, he gets off with a couple of wrinkles and gray hair in the fashion of Reed Richards, as he leads a mostly new generation of heroes who have similar powers to their predecessors. Aquagirl, Kai-Ro, Warhawk, and Micron, being the new Aquaman, Green Lantern, Hawkman/girl, and Atom respectively, have all made JL status and have shown to be somewhat experienced to handle worldwide threats with the decreased numbers compared to the League forty years ago.   

So the question this: When can a hero decide to hang up his cowl and utility belt for the last time? When is it time to let the younger guys handle the dangers that come with the symbol and powers on their own?  When is it possible for that hero to watch a burning building or a super-criminal terrorize the city and just sit in their recliner with assurance that there will be someone or someway the threat will be resolved?