i don't know how witty these responses are, but i tend to use one or the other of them when i'm just plain fed up with the "don't you worry about dying before (fill in the blank with any designated time, or event of significant importance in your child's life)".

if i'm in a snit i usually say: "Having a child when you're younger is not a guarantee of a long life."

if i'm really aggravated i'll say: "Young mothers die every day."

and if i'm really trying to go for the jugular i'll say:
"Young mothers die every day. Mine did."

Here's one:
I'm strong and in good health, and have no intention of leaving this world any time soon. Longevity runs in my family.

Here's another:
It is very rude and insensitive to talk about people's ages, especially in front of their children.

Hope I will come up with more and better ones before I get that "first comment."
Lisa

hmmmm, how about "yup, I am a grand, great mother, thanks for noticing!!!" Or from the child --- she is a GREAT mommy, not a GRAND mommy, silly!!

(that doesn't acknowledge the death issue, but sometimes its more of an issue to the grownups than it is to the children... if that barrier hasn't been broken -- as in seeing other people within your age bracket die -- then they may not realize that it applies to you.)