After doing some research and doing my analysis,

Here is my theory on the year of birth of the characters:

Marina (Coralie):

Marina would officially be 18 years old so she would have been born in 2006, in the 2000s there was a style called "Y2K" which consisted of having clothes that left the belly visible not covered by a T-shirt.

However I saw some information that says she would be 23 years old in Splatoon 3 which means she would have been born in 2001 instead of 2006, but since Nintendo has made the information official that she is 18 years old and that 23 years old is not official she would have been born between 2001 and 2006.

No worries I conducted the investigation to have a more precise year on her birth,

For that I looked at the year of release of splatoon 2 which is July 21, 2017, officially Marina is 18 years old in splatoon 2 so I did the calculation 2017 - 18 which gives 1999, in splatoon 3 she would be 23 years old "approximately" here is the calculation I did, splatoon 3 was released on September 9, 2022 so I did the calculation 2022 – 23 and the result is also 1999, so Marina was not born in 2006 or 2001 but she was born in 1999!

Pearl:

Pearl is officially 21 years old so she would have been born in 2003, between 2000 and 2005 approximately, the hip-hop and rap style was quite popular, however I also think that the dates can be from the 80s to 2010 at the latest on the period of popularity of hip-hop and rap.

So I did my research to see if this is the correct year or not,

Splatoon 2 was released on July 21, 2017, so I calculated 2017 – 21 which is 1996,

In splatoon 3 she would be 26, splatoon 3 was released on September 9, 2022, so I calculated 2022 – 26 which is 1996 as well, so Pearl was born in 1996!

Shiver:

Shiver doesn't have an age specified but I think she must be between 16 and 20 years old.

Why?

Because the style of ripped pants is often a style that can be had before or during adolescence.

Frye:

Frye doesn't have an age either, but I would say 15 to 19 years old.

Why?

Because her voice seems to be young and also in relation to the size, even if it is not necessarily on what is needed based on Pearl who is 21 years old while she is small.

Callie (Ayo):

Nintendo is not clear on their age, however it is said that she would be between 17 and 24 years old,

Here is how I thought about her age:

Splatoon 1 was released on May 28, 2015 so 9 years ago, so I calculated 2015-17 which gives 1998,

Why 2015 - 17?

You will understand why, the highest age is 24 years old and so I calculated 2024-24 which gives 2000, as a reminder splatoon 1 was released on May 28, 2015, for Callie (Ayo) to be 17 years old she would have to be born in 1998, although Nintendo does not specify it is therefore likely that she is 26 years old in 2024.

2022 is the year splatoon 3 was released so it would be logical that she was 24 years old at that time,

1998 = year of birth

2015 = 17 years old, year of release of splatoon 1 which makes my theory logical

2017 = 19 years old, which makes the age threshold more precise even if nintendo said "approximately"

2022 = 24 years old, which corresponds to the highest age in her age estimate

And so in 2024 she is 26 years old, so Callies was born in 1998.

Marie (Oly):

For Marie (Oly) Nintendo also said between 17 and 24 years old, which is the same calculation as Callie (Ayo),

Marie was therefore also born in 1998, which means that not only are Callie (Ayo) and Marie (Oly) sisters but she could also be twin sisters.

Why?

I understand that you are surprised because it is often said that twin sisters or twin brothers are the same with the same appearance except that this is not always the case,

It is possible to be twins and not look alike even if for me it is not easy to explain,

But to put it simply sometimes twins can be the opposite of their brother or sister,

Let me explain, Callie (Ayo) is energetic while Marie (Oly) is calm and balanced, which makes them twin sisters but one is the opposite of the other, according to me in medicine it can happen more often than we think that twins are the opposite of each other, however I am not sure, on the other hand I am 99% sure that Callie (Ayo) and Marie (Oly) are therefore twin sisters but one is the opposite of the other, just as "squid sister" indicates that they are sisters which makes my theory possibly true.

About Marie:

I have one more thing to say about Marie, if she knows an old style of Japanese song which is a mix of hip-hop and traditional song from memory it is therefore not a coincidence since she was born in 1998, at that time Japan was not yet as modern as today,

Which explains why Marie is often dressed in a cultural style of Japan from Splatoon 2.

To conclude:

Here is the list of the characters' years of birth in ascending order:

1996: Pearl.

1998: Marie (Oly) and Callie (Ayo).

1999: Marina (Coralie).

2003: Frye [probably].

2002 or 2006: Shiver [not sure].

Be careful, Frye and Shiver don't have an official age so I based it on the ages I estimated, for Frye I think she was born in 2003 because the 2 ages I gave her with my calculation gave 2003 while for Shiver the 2 ages I gave her didn't give the same year so I put 2 different years, I calculated by doing 2022 minus the estimate of their ages.

I hope you liked it!

Dylan Guizor