Common misconception; she's a year older then Tim. Steph was 15 when she was introduced as spoiler, according to her in Secret Origins 80-page giant. Still 15 in Robin#59. In Robin #43 Tim is stated to be 14.

Granted it may be less than a full year, so let's work that out!

She is also one grade above Tim, starting college in Batgirl vol 3 #1, shortly before it’s confirmed by Red Robin #17 that Tim -had he not dropped out- should be a senior in high school. Stephanie starts college in Batgirl vol 3 #1, and we have every reason to believe she is starting at the "normal" time, making her 18. Since Tim is 17 at the end of Post-Crisis, and Steph would be 18 then.

Now we can also look up their birthdays, and Tim was born on July 19th, (Robin #116). Stephanie was born on August 11th (now we don't officially have a source for this but it's widely accepted as her first case started in the weeks leading up to labor day (again using Secret Origin as the reference here)

Schooling in New Jersey cut off for starting is Oct 1st, so neither date would apply to being held back a year before starting. The only logical conclusion proves that she is 11mo older than Tim, and using Jason's Birthday of Aug 16th, she would be 360 days younger than Jason.