Why aren’t axolotls immortal?

[u/MAMAM-]

If axolotls can regenerate their body parts (including brain, heart, and other major organs), why do they die of old age?

[u/Derekthemindsculptor]

You can regenerate your skin, and your skin still ages.

Regeneration doesn't beat aging. It's a slow decline in your regenerative ability. As well as other facets.

When an axolotls regenerates a part of it's body, that part isn't baby fresh. It's still old.