What would this be?

[Lollipop_The_King]

Once you reach your physical prime, you will stop ageing.
Like, you can still die of old age, but your body doesn't wear down anymore past that point.

 

[SamanPatou]

That's hard and interesting, maybe it's just part of superhuman physical characteristics and should just be written for what it is without having a link to anything

 

[Shmooply]

Isn't that longevity?

 

[SamanPatou]

It depends on how much the person can live. They may live just as much as a human, but without changes in their body due to the age.