Some surprise ages here, but I got the main cast right being 15-16 and Nagi being 19 (because of Japanese student ages when they graduate high school). But with this, we now have an idea which part of the year Shin takes places.
Fret is 16 and born in April. Rindo is 15 born on October 20 (Japanese school system does not have students over a year apart in the same class). Both are in the same class. Now because of Fret's birthday, once you look at Neku, Neku's age doesn't make sense because he should be 19 unless Neku went from 14 to 15 in the year the events of the 1st game started (after his birthday) meaning he wasn't a 1st year high school student like Fret and Rindo are. But then you look at Bito being 18 after August 1st and especially Shiki's birthday is 8 days before Rindo. This creates a "WTF?!". But I'm just going to assume that there is inconsistency and Shiki's age was rounded up otherwise Rindo turned 16 during the game. But before I settle with this conclusion, Raimu being 17 born in January while Bito is 18 with a August birthday does not make sense and creates the biggest "WTF?!". Raimu cannot be 17 if Bito just turned 18 due to child birth reasons. The only time (within the game) they can be 17 and 18 is if Bito is turning 19 later that year (eg: Bito is born in August 1992 and Raimu is born in January 1994). Meaning Neku should in fact be 19 because of Fret. This makes Neku a college student and older than Nagi. With that said, September seems like the month that makes sense, especially with Fret not wearing sleeves, but he has a scarf and school is in session.
Now Neku being older than Bito is a surprise. Reminds me of Yugi (June birthday) from the Yu-Gi-Oh! manga being older than Kaiba (October) and Jonouchi (January the next year) despite being shorter and not physically stronger. Like I mentioned before, Japanese school system, so Jonouchi being born before April allows him to be in the same class as everyone from the year before (Up to April).
Just based on appearance in the original, I expected Raimu to be 2 years younger than Bito. The only surprise is that her birthday is in January so while the year being 2 years (eg: 1992 and 1994) is correct, her birth month makes it before a full 2 years.
Coco being 19 is a surprise overall based on her appearance and height (shorter than Raimu).
The waifu Shoka being physically 16 made me laugh. I expected it, but if you know, you know.
Uzuki being 17 is a surprise. I had her at 18. Kariya being so chill plus voiced by Anri Katsu, I felt like 20 was right, but 19 is close enough.
Susukichi being 19 makes sense. His face and being voiced by Hanae. lol
They forgot to switch Ayano and Kanon's age. lol Kanon looks like a older teenager (20 is the age of adulthood in Japan diring this games development).
The amount of "Gokigenyou! Shibuya no tame yo!" I heard (and got obsessed with) made me think Shiba was physically older than 20.
I think we can all agree that we thought Kubo was older. lol
I love that Neku and Shiki both have TWEWY release date birthdays.
Except Neku doesn't have a release date birthday. The NA release date is April 22nd. His birthday is April 21st. A mere coincidence. If you knew this when you said it and it wasn't a mistake then by your logic, Fret also has a release date birthday because his is on April 20th and the NA and Europe release dates are on the 22nd and the 18th (2 days after and before), respectively. Or Kariya having a release date birthday because Final Remix released on September 27th and his birthday is on September 26th. It's not a release date birthday no matter how much you try to force it.
The only characters who have a release date birthday (whether it was intentional or not) are Shiki (October 12: Final Remix release date) and Nagi (July 27: Subaseka and Shin release date).