Board Thread:Questions and Answers/@comment-30272238-20161020193809/@comment-29602226-20161020204443

The canon answer is that they never grow old (they always look young), but they have a limited lifespan. Even Succubi have a lifespan of centuries to thousands of years.

However, DE extends life, and there is Mermaid's Blood, that stops aging. And the spell of Pedomorphosis can keep Mamono eternally young. And even if they die, there is undeath...

In short, Mamono have a limited lifespan, but they have several ways to prolong their lives and keep themselves from dying of old age.