User blog comment:SodaCat/Birthdates According to Graduation Year/@comment-5385060-20160123055407

There is a pretty good way I use. I just decide when their birthday is. Figure out where the timeline is. Using last year for example;

Random character's birthday is June, 2nd. But my timeline starts in March. So it is set in 2015. Say they are 17 when the story starts, but become 18 in 2015. So working out their birth year would be the year before. 2014 - 17 = 1997.

Try it with your own age and birth year. It works. I tested my own using this method, and it was spot on.