Well, the most important factor is what they were in the past, because how much you were earning up until the time of the wrongful death is a good predictor of what you might've earned into the future. Of course, no one works forever, and people do stop working even before the end of their lives, so you have to show what the actuarial tables would indicate. These are government statistics that the government keeps of people, how long they live, how long they work on average. Those are the factors.
You can also show that in your particular case, the decedent was the type of person who would've worked beyond the average person, and would have earned money throughout that time frame. It's difficult to prove lost earnings in the future if the decedent did not have a track record, although with a young person that, of course, could be an exception because they're at the beginning of their career. If you can show that their schooling and inclinations and likelihood of a high income position was there, then you can argue that as well.