Sensei ghosted Niki 8 years ago:
Not at least 10.
Noriko is a few months older than Ami, and the accident happened when Ami was 7.
Sensei had likely been tutoring Noriko since she was 6, took a break when she was 7 due to the accident, continued to tutor her seemingly within months after it, and Maya had showed up, then he tutored them until about 4 years ago. So, until they were probably around 11 to 12. (Maya is one of the youngest).
Ami was 7 when the accident happened, and he's likely been with her since.
Niki was around 16, and Sensei 18, when they dated for 5 years, then the accident happened when Niki was 21 and Sensei 23.
After disappearing around 4 years ago on Noriko, she seemed to find Sensei again almost 3 years ago (on a Thursday) and something happened, but it's not clear.
8 years after the accident and ghosting Niki, is the present. Sensei is 31, Niki is 29, Noriko is 16, Ami is 15 shortly before her bday, then she's 16, and Maya is 15 until New Years Day. They are also in the year 2020.