I would like to point out; we have no idea how old Shisui was when he graduated from the Academy. At the time, the average age was nine (see Obito Uchiha and Rin Nohara). We don't know whether or not Shisui graduated early. That plays a key deciding factor here, considering he met Itachi approximately two or three years after he graduated.
If he graduated at the average wartime age, this would make him somewhere around eighteen to twenty years old at the time of his death. Thus, making him six years older than Itachi, give or take. Perhaps he graduated at six, like Kakashi, or maybe seven. Possibly eight. We have no idea. We don't know how long Shisui fought in the war for either. What if it was a year? Or more?
However, gauging his visual age 18-20 makes the most sense. In addition, have you seen the difference in height between Shisui and Itachi when they first met? There's too big a difference for there to simply be a three-year age gap, in my opinion.
Also, you need to factor in Shisui's height and weight stats from the databook. At the time of his death, he had the height and weight of an average adult male. He couldn't have been less than eighteen. I like to believe he was nineteen; six years older than Itachi.