I'm about three months short of 84. During my life I spent years designing solutions to military problems, wrote a bunch of poetry that was published in "little" magazines, for several years wrote on politics and economics for magazines like The Freeman, wrote some fiction that wasn't published, though there are some short stories resident at
http://www.russ-lewis.com/asia/Shorts/S-preface.html. I did software engineering for thirty years after I retired from the air force and designed and built some fairly large systems. for about fifteen years I've built websites -- mostly for the fun of it, and recently, after a lapse of five years, I got back into programming and built some pretty fun software in C#. I've been an amateur photographer since I was twelve, and beginning at the age of 23 I got serious about it and started doing the kind of work that got me occasional publication and an occasional award.
I'm not sure how you measure "creativity," but all my life I've found great joy in creating things. The urge hasn't gone away. What's gone away gradually as I age is the ability to recover the words I want. I like writing on the web because I can stop and use Google to overcome that problem. I used to be able to stand up in front of a large group and speak comfortably and extemporaneously, a skill that stood me in good stead when I was a staff officer, a unit commander, and when I was mayor of Manitou Springs, Colorado, but now I often go for a word and can't grab it. I've had to stop trying to do it.
I don't agree at all with Doris Lessing. There always are slumps -- temporary burnouts -- in anyone's creativity, but the desire to create doesn't go away with age. What happens with age is that your toolkit changes. Some tools begin to shrink and become less useful, but the most important tool of all -- experience -- continues to grow.
Don't sweat it, gang. Keep shooting.