Won't let me choose children as heir

[Conan1600]

So love the game though some of these features left out Hurt but the problem I'm facing atm is the game won't let me choose my son as heir when I die. He is 22, in my party, low skills to be sure but he is there. Give me an option for Varic, Alda or my wife but not my son. What did I do wrong here and how can I fix it

 

[Figulus]

Likely nothing..that part of the game is undeveloped atm.