TBH, Allison is in my age bracket. I not only don't find her too old, I find her VERY HOT. My Ex-wife is one year older than Allison, so...
The Great Grandma, well, fuck it, the lady wanted a ride on Zeus", who ma I to say no... I only got a "gum job", so far, but, if she wants more, I'll help her go with a smile on her face. After all, I (me the player) have Lost NOTHING.
I understand from these objections that you are stuck on either the roles, or the ages, comparing them with your IRL experience. I will be the first to congratulate you on your instincts and your love for your Nana. But, you have to understand, Allison, is not your Nana, it's OURS. And she's Awesome.
Peace