She's worked out that I'll be sixty-eight when she finishes college. She's 10

She did the maths at dinner, out loud, then asked whether I'd 'still be okay' by then. My partner laughed it off. I didn't have a good answer and said something vague about walking every day. I had her late and I don't regret it for a second. But she's clearly been thinking about it, possibly since a friend's grandmother turned out to be younger than me. How do other older parents talk about age with their kids without either lying or frightening them?