Uh-oh.......
A little research turned up
this and
this, which confirms my idea that Mikoto, being second year middle/junior high school, would be 13.....and Kuroko being a year/grade younger/below is.........12.
My eyes might bleed the next time I see her.
Maybe Toaru uses a special and unique age-to-grade system different from the standard Japanese system?
I guess this could make sense actually, since it's
Academy City, the school capital of the world.