When should I talk to my parents about end of life decisions?

Anytime. Shortest answer is anytime. Do you want to be kept alive with machines when there is no hope for recovery, fed with a tube in the stomach, and who do you want to make your decisions if there come a time when you cannot? What are your hopes and goals for your older age years? These are tough decisions and should be given time to consider, so anytime is a great time.
Early. The earlier the better when they are being realistic of their decisions also having a power of attorny & knowing their wishes frequently helpful.