if she is 500 years old she would have died misery is like 12 years old
Misery is the daughter of Jenka, who is the sister of Ballos, the one who went mad with his power and had to be sealed away.
When you're the daughter of the witch that made a whole 'island' fly and seal Ballos on it, immortality, or just long lives in general, might not be that far away.
And even then it might be possible that Misery and Balrog have been put in some kinda stasis while waiting for the next crown holder (see: the two flying in bubbles in the intro), leading to her being chronologically old but physically young.
And even then theres always the possibility that the events from cave story from the sealing of Ballos to his destruction by Quote have been within the lifespan of both Jenka and Misery. Which isnt too hard to assume since there are only 4 statues of demon crown bearers. I don't think theres any info on how much time has passed between crown holders aside from between the Doctor and whoever was before him.
Regardless, wether Ballos/Jenka/Misery are (partially?) human or something else or have achieved a long lifespan or immortality, it's very safe to assume that Misery is way older than Quote, even if just chronologically.