Certain posters are not going to like that viewpoint!
I think looking at his brothers progression we might understand how things may pan out for Jakob.
Henrik ran 3:44 at 18, 3:38 at 19, improved that to 3:33.9 by 22 and at 23 ran 3:31.46 which is still his PB.
Filip ran 3;44 at 19, 3:38 at 20, then 3:32 at 23 and then took that down to 3:30 at 25 which remains his PB.
Jakob ran 3:42 at 15, 3:39 at 16, then improved to 3:31 at 17, and then 3:30.1 at 18
It is difficult to draw a pattern.
One thing that is clear is they took 3-4 years to build to elite athletes after breaking 3:40, and then rapidly in 1-2 years reached their lifetime bests.
Jakob's improvements seem to be shortened into a much smaller time frame than his brothers. Likely due to the fact that he trained with them at a high level whilst young. He has rapidly improved from 3:39 to 3:31 and then a more gradual 3:30.
It looks like he has reached a bit of a plateau, but I'd expect he runs faster this year or next and then chips another second or two over the next few years. Maybe he will peak around 21-22 with a 3:28 time.