Each time something happens to me that reminds me that I'm heading into the third act of my life, I am startled and a bit annoyed. It's not that I didn't know these things would happen -- the memory lapses, the slight hair loss, tiring more quickly, sleeping less soundly -- I just thought they would happen, you know, later.
Whether the perfect balance exists or not, one thing is clear: it is all part of the journey. Decide what is most important to you and make it the priority. For me, balance happens when I am present; when I am having fun while still striving, and when I am able to marry my priorities with my passions.