Well, most lore about elfish races is they live very long lives; sometimes a thousand years or longer. Others its just a few centuries, but still, usually elves live longer than humans. So seeing elezen not aging within 20 years isn't odd to me at all.

We also don't know lifespans of Hyur, Miqo'te, Roegadyn and Lalafell. Longer lifespans means signs of aging take longer to appear, shorter means its more noticeable.