It says in cartoon it was on him for years. In most of media Superman ages very slowly if he ages at all to begin with. Was Starro leeching of Clark, the reason he started to visually get old and get grey hair? Was getting rid of parasite, will rejuvenate him and get him young again?
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No. We see the moment Starro attaches itself to Superman and he’s visually the same age as when Batman and the League discover it years later.
His aging is slower than a human’s. He and Bruce are rooooooughly the same age and Clark looks like he’s in his forties while Bruce is very decidedly in his eighties. It’s faster than some other depictions where he’s virtually immortal, but it’s not unheard of. Kingdom Come, probably the definitive older Superman story, depicts him as being in his sixties and he looks pretty much the same as Beyond’s Superman.
DCAU Superman is also generally on a lower power scale than a lot of other depictions, so it makes sense that he isn’t one of the immortal ones.
I’ve now said Superman so much it doesn’t feel like a real word.