In an alternate Joker origin, Lovers and Madmen, Harley's a pre med student going on med during the time Batman finished dealing with mobster and Joker's debuting. So she's already in college. Harley mentioned 8 years of education in a recent story, forgot which, after that she did around 3 years of apprenticeship that's how you get the license, the last one should be in Arkham.
11 or less years from Batman Year One will land her during Dick's late high school or early college years when debuting as Harley Quin.
So she'd be about 8-10 years older than Dick, 6-8 years younger than Bruce. Currently late 30s.