My Mum had me at 40, I'm now 47, she only died last year, still fairly mobile and 'all there', she worked until she was 80, as did my Nan and babysat my children, even overnight, in her 70's.
I think it is fair to take into account of how long people tend to live in your family and any health issues.
I'd hate to have a young child, my youngest is 18 and I've got loads planned. The sort of, lack of responsibility, is exciting, its different from when you are younger, because you have the confidence to do what you want and not care about inconsequential stuff.
I love the relationship that I've had with my older and now adult children, another younger child would have set that back.
I don't know your circumstances, but I enjoyed (for want of a better word) the last months that I spent with my Mum, taking her out etc.
It's a very personal decision.