Goodness, 40 is'nt old! I agree it depends on attitude, genes, decisions about parenting, and probably luck. My grandmother is 101 (and has only had a carer the past year and a half) and my mother, just turned, 80, travels every month to Paris to look after her for a couple of weeks. My father is 82 and travels all the time, and until recent brush with prostate cancer (now apparently gone) could outclimb, and outswim, me.
I had dd1 at 43, and dd2 at 46...now, I didn't want to have kids at this age, I'd have loved them younger, but it didnt' happen in spite of all attempts. OTOH, we have more time and energy to spend on the dds than we would have had ten years ago, and don't regret a thing - time will tell if they mind or not!
We've got some grown up nephews and neices who provide young adult company for our LOs, and equally we have some young ones so there are same age cousins. The dds have godparents (of the non-religious variety) so there are plenty of trusted, loving, responsible adults around should the worst happen to me and/or dh. And I will book us into a retirement home well in advance of need so the dds needn't feel burdened....