She was rather sad about life closing in on her, and she's right. It is, as it is for most old people. (They can't all be the the grandma who still drives and does meals on wheel at age 105.) All I could say, is you right, it is, and it can't be very nice. Is there something more positive I could say to her?
I know its very hard sometimes. I don't think its only the elderly. I'm sure a lot of middle age people and others dealing with loss also have this question about life closing in on them. It is hard to know what will be comforting and helpful to say. Even with my own parents and grandmother, I sometimes found I didn't have the patience to even listen to their true feelings on the matter. I would try to validate her feeling and like the others have suggested then redirect her thoughts to happier ones if you can. Or ask her more questions like "why do you feel this way? Or what would like to do that will make you happy?" Maybe all they need is more conversation. I don't know your mom's exact age but I was reading about a lady at 89 years old that has a bucket list every year. She really inspired me.