Yeah, it pushes the boundaries of the characters' ages, but I'm trying to base as much as possible on my life, so I can relate to the story more and keep it as truthful as possible (I turn 24 in July and my dad would have turned 80 in March). As for the frailty of the character, that's something that has been bothering me a bit.
Kudos to your Dad then...I am guessing by the use of past tense that he has passed away, so please do not take anything I say as insulting to his memory. I like the fact that you want to keep aspects of your storyboard close to your own reality of yourself and your father. It will help you keep fighting to make this more than an idea. But and this is a big but, don't lock yourself on it so much so that you have no flexibility. A project on the scale this appears to be heading to, will most likely result in a team-based effort and you need to keep that flexibility for their input. On the other hand don't be so flexible as to lose what you consider important.
One aspect you might look at is the actual relationship dynamic being a lot more intense than you might initially think of right now. What if the value that the father provides is an oral history of his own life and in that oral history you literally have a game within a game? This life affirmation could also be used to foretell the backstory to the societal collapse as well by providing a rich history that give absolute credence to the age gap. The sheer desperate moments of the day to day survival leavened by the alternate game within a game could prove a rather interesting game.
edit note: I said history and game waaaay too much. ><