So I’ve done the math to try and figure out how old Kili and Fili were in human years when they died. They talk in the books about how young the were, but it is a little hard to grasp when everything is in Dwarf years.
In Dwarf years, Fili was 82 and Kili 77. Now if you take the average age of Dwarf when they die (340) and divide it by the average age of human (80) you get 4.25. Now divide 82 by 4.25 and 77 by 4.25 and you get their ages.
Fili was 19 when he died. Kili was 18.