Not my point. If Momo can be the ruler of a Kingdom 10 times the size of Zou at 8 years old, then I don't see an issue with the Dukes, who seem to be versed in Wano customs, appointing Carrot the ruler of Zou at 15. If they are going to be serving an 8 year old ruler then obviously the age of a ruler is not something they weigh heavily.