A Reality of Middle Age?

I've, of course, never met that special someone I wanted to settle down & raise a family with. And I used to BURN for that. Big decisions were made based on that desire. Now, in my mid-40's, that just doesn't seem so important. Living the kind of life I want and experiencing the joys and wonders of the world now seems more important than settling down, staying put, and raising kids. I wonder if that's biological? While I'm technically not past parenting age, many of the people I went to school with now have adult children. Some of my peers are even grandparents. I can't imagine. It's not that I'm adverse to the idea of settling down. It's just no longer a top priority. Thoughts?