Would it be okay if I told my future mother in law that she can't see her grand children in the future?
Why would want to keep your kids from her?
Because I can't imagine a lady older than my mom being a grandma
That seems like a petty reason to keep them away. A grandmother plays an important role in her grandchild's life. You should not deprive either of them from that opportunity.
No I'm going to tell her that her future grand kids can't see her. She's just too old!
This makes no sense what you are saying, and while it is a little bit quirky it would be considered a terrible reason to keep someone away from their grandchildren. How would you feel if you were kept away from people that you love for a silly reason?