I have recently played EU4 (a Strategy game) and noticed that if you play as one of the remnants of the Mongol Empire which still has Khans/Khagans, your heir can't rule until their 15th birthday and will instead be replaced by a random clan-member.
I searched the internet and couldn't really dig anything up on the topic, so I'd like to know whether this has any historical background or if the developers just invented that mechanism for the game.
TL;DR: What was the age requirement to rule a Khanate/Khaganate? Or was there even any?
Thanks in advance!