Are you saying there is a floating jelly fish in the ocean that is 500 million years old (‘years old’ in the common parlance of usage)?
Only via predators. The immortal Jellyfish not only don't die from aging, they don't generally die of disease, starvation, temperature changes or any other stresses...in fact it is those stresses that trigger their reversion from sexual maturity to biologically rewind to a polyp colony where they begin the start of their life cycle all over.
I tend to agree statistically speaking it seems highly unlikely, but its such a bizarre thought experiment, I can wind up concluding it is statistically highly unlikely there isn't Jellyfish in the wild that are older than the dinosaurs. Unfortunately, we can't age these Jellyfish.
Unfortunately we can't age these Jelly fish or even confirm they have undergone the reversion transformation process (except under lab observation).
So while I am not saying yes, a single jelly fish has lived the entire 500 million years, its certainly possible and the idea behind biological immortality. Maybe one day we can figure out a way to age them and find some older than the dinosaurs.