Time is flying by for my grandsons, too - my oldest grandson will be a teen on his next birthday. This seems impossible, but it is not.
It may be easy for us to think our "best days" are behind us, but while there are certainly many more days in our past than in our future on this Earth, and many of them may have been wonderful days, as long as we are living, God has a plan for us. He wants us to continue to live each day as if we are still "in the race", because we are.
As long as there are days before us, we need to be committed to make the most of every day we have, to finish the course, and to finish it well with our all. We need to display the "Gospel of the grace of God" in all we do and run our race to the end - whenever the end may be. When we do this, we will continue to have many "best days" before us - no matter our age!