Timeline

Working out when Saori was transferred, when she came back, and where she might have been when game events played out.


Saori: I'm actually two years older than you. I studied abroad, and took some time off from school.

Since 17 is the unanimous age for 2nd years, that would make her 19 in current game time.
There's no exact date for when Saori was transferred, but she gives a little hint in her first Sunday/Holiday date, when she and FeMC are eating out at Wild-Duck Burger.

Saori: I came to places like these fairly often until about middle school. I'd study with my friends there before exams.

This means Saori *did* have friends up until middle school. Her being the oldest student is what makes everyone interacting with her uncomfortable, so we can assume up 'til then she was the same age as her classmates. Meaning she was transferred abroad sometime in middle school. The "about" implies it was sometime in her first year, so she was likely 12-13, and 14-15 when she came back.

The game takes place in 2009, so subtract her age and you get 1990, her birth year. There's no official birthday for her otherwise.

1990 - Born
1999 - The Kirijo Lab incident (9 years old)
2003 - When she was transferred (13 years old)
2005 - Her return from abroad (15 years old)
2008 - Entering Gekkoukan High as a freshman (18 years old)
2009 - Game plot (19 years old)

So that's a rough outline.