You can do it after he's 6 years or 72 months old, but it will cost 10 GB.

You can do it for free when he turns 15 years or 180 months old.

When he turns 16 years or 192 months he will die no matter what, and you'll be forced to chose a heir.

But now some of you might be asking "When can i get rid of my crappy rolled lion?" It depend on his age. My king is 9 years old, i'll use him to show you. Let's pretend that i want him untill he dies at 192 months.

9 x 12 = 108 So my king is 108 months old.
192 - 108 = 84 He'll die over 84 months.
84 : 12 = 7 I have to wait 7 years (lioden time) for him to die.

Now some might be asking "What about my heir?" again, that depents on the kings age. With the math i just did i already know that my heir will be 7 years and 11 months when i crown him. But i'm going to retire my king as soon as he's 15. More math!

9 x 12 = 108 Again, 108 months old.
180 - 108 = 72 Now he'll die over 72 months.
72 : 12 = 6 I have to wait 6 years (lioden time) for him to die.
So my heir will be 6 years and 11 months when he becomes king.

But if you're heir is 10 years or older they will age down to 8 years once he's king.

If you're to lazy to do the math yourself just post a comment here or PM me with your kings age and your heirs age. And don't be afraid to tell me if i did something wrong.

Hope this helps.