I guess it would take some getting used to, but it seems to be not uncommon these days, with remarriages, etc.
My stepsons are 24 and 21, and the 21yo was on a kick for a while that he thought his dad and I really, really needed to have a baby! Well, that would require some medical intervention and a lot of wishing at this point (as well as a lot of risk since I'm 47 and filled with uterine fibroids), but it did make me think about what a huge adjustment it would be to take on the role of parent to an infant at this stage in life.
Here's another perspective: I'm the late child of older parents. All of my grandparents were gone before I was born, except my maternal grandmother, who died when I was still very young so I have no memories of her. My dad passed away when I was 25, just when I was getting to the age where our relationship was becoming less of a parent-child dynamic and more equal. Dad was a pretty reserved man, so I felt like I was only starting to get to know him when I lost him. These are just a couple of unfortunate realities of being a child of older parents, but there are lots of good things too. I know my parents were thrilled when I came along, and they were much more relaxed in their parenting of me than my sister since they were older and wiser by the time I came along.