My "theory" is based on what happened to Luka's Croatian family:
In the "bishop storyline" in Season 7 we were told that Danijela and the kids were killed in the siege of Vukovar, which is a historical fact, and it happened in the autumn/fall of 1991. At this point in real life, Goran was 18-19 years old.
In "Secrets & lies" we know that Luka and Danijela married young, but did they really have their two children while still being teenagers? I believe they were older. We know (in "Caņon city"?) that when he died Marko was at least old enough to walk, as he had once got lost on the beach and walked back home, so the children weren't exactly newborn babies when they died.
All of this combined makes me think that Luka is older than Goran. Any thoughts on that?