Orson Welles:

It’s only in your twenties and in your seventies and eighties that you do the greatest work. The enemy of society is the middle class, and the enemy of life is middle age. Youth and old age are great times — and we must treasure old age and give genius the capacity to function in old age — and not send them away. 

M. John Harrison: 

The idea you have when you’re young, to reach the edge of what can be done with your abilities and find out what might happen if you went past it? You promise yourself you’ll try but then wake up fifty years later to discover that you were in fact always too sensible to push things until they fell over, in case people thought less of you. In your seventies, though, it doesn’t seem to matter any more what other people think. That’s probably the first phase of your life in which you can actually do what you want. And certainly the last.