I don't think this necessarily is an age problem. I'm twentyfive and although I don't have this problem so frequently anymore, I can recognize it... I used to forget absolutely everything. I forgot that I had to college, and I forgot to pay people back when I owed them money. People thought I just didn't have the money, or didn't want to pay them back -but that wasn't it, it had just dissapeared from my mind altogether.
I've been told by my therapist that this isn't uncommon for people with traumatic experiences. There's a lot of things going on in your brain after that (even five years after it, as was the case for me) so there seems to be less space left for your brain to remember other stuff.
Nonetheless lately I've become better at remembering things and I noticed just now because I read your post. I actually remember people's names sometimes, for example. I used to forget to write things in my agenda and I used to forget looking in the agenda, too. I don't really now why I am making progress now, but one thing that helps: write it on a post-it immediately (don't wait!) and put it somewhere you will always see.
Oh, and explain it to your grandson, if you can! it's important to help people understand, otherwise they will fill it in for themselves. And since they don't have this weird forgetting thing, they might think that you forgot because you don't care, which obviously isn't true...