Old?! I am not!

I watched the movie, 17 Again, about a 40-something man who magically gets transformed back to the age of 17.  There's a scene where the younger version of the character (Zac Effron) looks at his body in amazement and exclaims how great he feels.  I had never consciously noticed not feeling good anymore, but when I started thinking about it, that's probably a fair assessment.  When did that happen?  Then the next day, I went to yoga and there were poses I used to do in high school every day before practice without a second thought but when I attempted the pose two days ago, my body said, "uh, old lady, you haven't done this in 20 years, are you kidding me?"  How fitting that I have a birthday coming up. I'm kind of past being depressed about the actual number changing, I'm really more sad about the time I have left running out.  I read somewhere about the possibility of humans living (healthily) to 140.  One hundred more years!  I would have no excuses not to do everything I want to do!  I think I could still do most things I want now but I find myself dismissing doing them on account of  "I'm too old."  I'm too old to start a business, I'm too old to make a career change, I'm too old to learn a new language, I'm too old to have a baby, and the list goes on.  Then I ask myself, "When did I become old?" because, truthfully, I don't think of myself as old.  Maybe I'm not really too old, but damn, I am too tired.  Since the big day is fast approaching, I can say with certainty that this isn't the last entry about the subject.