By definition at least half the lords in a close battle will get wounded (defeated). If they fight 50 losing battles before their heirs reach 18, which is ~3 losing battles per year, they, on average, will die before they become adults, so if we believe they will fight 50 losing battles in 18 game years, which to be honest, it is pretty likely, we will end up with no lords in no time as younger people will take time to marry and have children themselves... (or at least no lords on losing factions, which will tend to get outnumbered and killed in battles as lords die and armies become smaller)
50 battles * 2% dying change = 100% chance of dying (on average)
By the way, I think there is no children/adolescents, they should add that to smooth that transition...
Yes you are right even 2% is a huge probability. I am not owner of design at these parts but I suggested reducing death probability 5x when I see it is 10% currently. I do not know why that huge probability is decided at first. It was 10% before and as a bug it was not added to simulations, maybe we can make it even 1% after seeing results of long term tests.