Does it ever end? I liked Janet's approach...
If difficult child wants to spread his wings when he gets to 18, we'll have to let him... but it will be a long road with many crashes, and we'll have to learn new skills - how much help really helps and when to back off.
If difficult child wants to give himself time to mature, so that his brain catches up with his body (it will if he lives that long), then he'll be around until at least 22, maybe 25.
By then... I'll be almost ready to start cashing pension cheques. <if I have any pension to draw on...>