So it turns out that the character ages actually update as the games progress to reflect the time of year. It's hard to track in English though, since the ages are ripped out of the English versions.
I knew we had different ages listed for Timeline 1 and Timeline 2, but I hadn't really looked into it before. It's apparently because Deep Crimson takes place immediately following the events of Burst in Timeline 1, but Shinovi Versus takes place 6 months later in Timeline 2.
Due to that 6 month difference, several of the characters are older due to when their birthdays fall.
New Wave actually came out a few months before Shinovi Versus and uses the same ages as Burst for the characters that appeared in that, and plugging in the ages from Gessen and New Hebijo seem to fit in regards to who changes and who doesn't, so that's how I was able to include them.
I'll list their birthdays in order, starting in April since that's the start of the Japanese school year and we know all the games are the same year because nobody has graduated or moved up a grade.
- April 5: Yozakura
- June 9: Imu
- July 7: Ikaruga
- July 20: Haruka
- Aug 15: Miyabi
- Sept 8: Asuka
- Sept 9: Hikage
- Oct 8: Murakumo
- Oct 26: Murasaki
- Nov 5: Katsuragi
- Dec 23: Yagyu
- Dec 28: Mirai
- Dec 31: Yumi
- Jan 3: Homura
- Jan 19: Ryobi/Ryona
- Feb 10: Yomi
- Feb 14: Minori
- Feb 18: Hibari
- Mar 25: Shiki
Going from Burst/New Wave to Shinovi Versus, the bolded characters went up a year. It also is pretty much a six month gap, which is the period of time we know took place between Burst and Shinovi Versus.
So Burst started some place between July 20th and August 14th, and Shinovi Versus started some place between February 10th and February 13th of the following year. I say started because the games don't update ages as you complete missions, and it's entirely possible for these campaigns to span longer than a few days, so figured everyone's stated ages would be at the start of each game.
So assuming all the birthday information is correct, we know Shinovi Versus starts on February 10th or later (as Yomi has her age go up), but before February 14th (as Minori has not had her age go up). This would be backed up by the statements in Chapter 1 of the Hanzō storyline saying Valentine's is coming up. Additionally, Hibari's birthday is actually celebrated during the Shinobi Girl's Heart missions, which generally seem to take place after the main missions.
As a side note, the fact that nobody probably celebrated Yomi's birthday (as Crimson Squad is pretty poor, even if they knew it was her birthday which they may not have) gives extra weight to her Shinobi Girl's Heart missions being about going on a rampage over people having too much extravagance in their lives, but then being calmed down by Minori's treats.
Anyway, back to the main topic, I don't have the ages for Estival Versus and Peach Beach Splash, however we know it's still Winter during Peach Beach Splash, as it's explicitly stated in the opening and ending cinematics. Since the first day of spring is March 20th, that should mean that only Minori and Hibari's ages changed since the next birthday isn't until March 25th with Shiki.
That means Estival Versus and Peach Beach Splash both take place some time between February 18th and March 19th, though I want to do some more checking to see if there are any reference proving/disproving that.
This also boots the Shinovi Master anime out of being able to possibly be in Timeline 2 as takes place before Christmas but still references events from various games as having already happened, including Peach Beach Splash. So it'd be in it's own timeline somewhere where similar events happened earlier or something.
I don't know enough about New Link to know if it's storyline takes place in frozen time like Estival Versus or what, but I know it has a lot of story, and so if characters aren't graduating then it might be in it's own timeline instead of continuing Timeline 2, because we're getting pretty close to April when characters would graduate.