Mermaids are mythical creatures that appear like perpetually young women. But do they actually age and exactly how long do mermaids live? Here is what I found about how old they might get.
Mermaids live very long lives but are not immortal. According to Greek mythology, they can get thousands of years old. Hans Christian Andersen also wrote in “The Little Mermaid” that mermaids live 300 years long until they die and turn into sea foam.
Let’s have a closer look at the lifespan of mermaids in mythology as well as from a biological point of view.
How long do mermaids live in mythology?
Since mermaids are mythical beings it is hard to define exactly how old they can become. But we can find some clues about their lifespan in Ancient Greek mythology as well as newer texts.
1000 year old mermaids from Greek mythology
The mermaid myth started in the Ancient Near East and became most prominent in Ancient Greece. The mermaids as we know them today are mostly based on how mermaids were depicted in Ancient Greece.
That’s why to find out the age of mermaids, we should look at what the Ancient Greek myths say about how long they live.
Mermaids were called Tritonides  after their merman father Triton the sea god. They were sea nymphs. And nymphs in general (lesser female nature deities) were known to reach around 1000 years of age . They were not immortal like some of the higher-order gods and goddesses of Greek mythology.
Some of the nymphs’ lifespan was actually tightly linked to the place in nature they guarded. This could for example be trees (tree nymphs: Dryads), springs, rivers and lakes (water nymphs: Naiads) or mountains and grottos (nymphs: Oreads). Once this natural place ceased to exist, the nymph that lived there also died.
For example, if the spring of a Naiad dried up, the water nymph would not have any life energy left as she was interwoven with the force of that spring.
This however also means, that it was possible for the nymphs to outlive 1000 years if their trees, springs etc. existed for longer than that. In fact, the Ancient Greek writer Plutarch suggested that the water nymphs (Naiads) could live longer than 9720 years. 
Now, the Tritonides did not watch over the ocean in the same way the Naiads watched over rivers and springs. Also, the ocean does not cease to exist after a thousand or so years like some trees or lakes might. Instead, the Tritonides were mostly guiding their ocean god and goddess parents on their trips throughout the sea.
The best guess at their lifespan we have is thus the fact that they were nymphs. And since nymphs were, in general, said to reach around 1000 years of age, we can assume that the Tritonides (mermaids) could also get around 1000 years old.
Hans Christian Andersen’s 300 year old mermaids
Another piece of literature that tells us how old mermaids can get is the famous 1837 fairytale of The Little Mermaid by Hans Christian Andersen. This fairytale definitely defined how we think about mermaids, especially since it is also the root story of the presumably most well-known mermaid ever: Disney’s Ariel.
In the original fairytale, the Little Mermaid talks to her grandma about how much she would like to see the human world outside of the ocean.
The grandma tells her about how they differ from humans: mermaids do not have an immortal soul and die at 300 years of age. After their death, they then turn into sea foam and rise up to the surface of the sea.
The grandma then presumes to talk about how the mermaids have a much better and longer life than the humans (who have that immortal soul and go into heaven after death). But this does not weaken the Little Mermaid’s wish to become human as we all know…
How old could mermaids get from a biological point of view?
As we have learned, mermaids can get at least 300 years or even around 1000 years old. But that is from a mythological point of view.
What if there were real mermaids out in the oceans? How old might they get from a biological point of view?
To estimate how long real mermaids might live, we can look at the lifespans of similarly sized and intelligent marine animals.
For example, most sharks get 20-30 years old, dolphins and manatees can live up to 60 years and the Greenland shark can even reach a staggering 500 years!
Similar to mermaids, the Greenland shark is commonly around 6.5 and 13 feet (2-4 meters) long and very intelligent. But why do they live so long and could mermaids also get that old? It is believed that the reason for their extraordinarily long life could be the cold waters they live in around the polar region of Greenland.
In response to these cold surroundings, their metabolism might slow down so they don’t use up too much energy. This slowed down metabolism might also delay the aging process significantly so that they simply age much slower.
But for mermaids to live that long, it would mean that they also would have to live in the polar region in ice-cold water. However, I don’t think that would be the most likely place you would find mermaids, as I described in this article about whether mermaids could be real.
The more likely answer could be that real mermaids would live in warmer waters and reach a similar age to humans (70-100 years) and marine mammals like dolphins and manatees (60 years). So, they would probably have a lifespan of around 60-100 years.
 Theoi. Tritonides. https://www.theoi.com/Nymphe/NymphaiTritonides.html
 Britannica. Nymph. https://www.britannica.com/topic/nymph-Greek-mythology
 Frederic Stewart Colwell (1989). Rivermen: A Romantic Iconography of the River and the Source, p. 48.