Half life?

I turned 50 in November 2008. Half a century! Earlier that year, I had been told by the faculty of a well-known southern "Ivy-league" university that I was "too old" to get a PhD. That's the same thing as saying that I am too old to
expect
dream
wish
hope
change
learn
wander
believe

I began to think about the criteria by which we evaluate age. If 45 to 50 is a mid-life crisis, then are we expected to live until 100? I really hope not. It's been a pretty good ride so far, but I can't imagine another 50 years of this. :)

But seriously - when are we too old? My colleague is still working and he is 80 years old. My Dad just bought a new computer for himself, and he is 85. Mom is a healthy 81 years. Yet my sister, who is 57, is struggling with oral cancer. Several of my friends have already died from accident or illness. If each of these people had known the time of their death, would they have made different choices? I'd like to think that we all live life to its fullest every day, but the reality is that we tend to coast along until we are shaken into some sort of awareness that our time draws nigh.

Despite the urgings of that southern icon, I have decided to go back for my PhD in 2009. I am prepared to work hard, but plan to enjoy the process of learning - now that I have some idea of what I want to do when I grow up. Some of my fellow students are older than I am. I'm hoping that no one told them that they were too old to reach for their dreams.

When does anyone have the right to tell you that you are you old to do anything - based simply upon a chronological event? If our ability is individual and circumstantial, then what, exactly, is the half life... of life?