I can't find Niil's back story anywhere, but yeah, he would've been 17 or 18 when Dhoresh got him out of the Academy. They were on the run together, but not for very long. Dhoresh didn't even have time to teach Niil how to properly tap into the Force; he probably only had time to pre-prep him about the Jedi philosophy and talk to him about the meditative state. Or maybe he had to calm Niil down before actually teaching him. Niil may have had trouble with tapping into the Force because he was still devastated and infuriated over hearing about the execution of his parents, and shocked to hear he was actually Force-sensitive. Trying to learn the Force in that state of mind would have been dangerous. So, with all of that, they might have been on the run together for a few months before the hunters killed Dhoresh.
So . . . figure 18 years old when he was taken from the Academy, another 2-4 months on the run with Dhoresh, and maybe another month or so with Dhoresh's smuggler friend before he was dropped off on Karvis III. And however long Niil was on Karvis III.
