I don't see how this can be solved if the goal is to age at the same rate.
If the rate will be as slow as the grownups, you will probably end the game many times before ever using your first child in combat.
If the rate will be as fast as the children, and you will want a realistic impact of age on the characters, it will only add weaknesses to heroes (who aren't that good now anyway). And it's not just the sovereign that has an age, it is also each and every hero.
What will happen to our motivation to raise champion's levels if we will know that when we finally raise him to a decent level he will die of natural age? Will we really want essence upkeep for each champion just to keep them alive, when all other ageless regular units keep gathering XP the usual way?
Best solution I can think of: remove the age entirely.
If it isn't realistic anyway, why bother with an inconsistent statistic that doesn't impact the game rules?