Have you ever heard that before? "Oh! These are the best years of your life!" What are you suppose to do with that? Some people think their teens were the best - carefree, no responsibility. Others say their 20's - "the world is your oyster." Maybe it is the middle years - raising a family, building a career. Or retirement might be the best - traveling, no children needing to be raised.
I think some of these concepts are really more mythic. Every person has varying experiences and when we are taught to believe that "this is the best" we set ourselves up for confusion and disappointment when it really isn't all that great. Life certainly has its own ebb and flow. So no matter your stage of life, it can fluctuate from wonderful to painful very quickly and then back to wonderful again.
How about looking at the moment? It is all you have anyway. Living each moment, intentionally, can bring about a great sense of being in the present and escaping the fantasy of what your life should look like. Whenever we live in the "shoulda, woulda, coulda" mentality, we aren't living presently. This moment is all you have. Enjoy it, understand it, and be in it.