Since aging is a life process that we cannot avoid, yes we can delay it. The most important thing to understand that is one need to implement the delaying acts while one is still young, so as to stay younger for longer. The following are the most effective ways of delaying the effects of aging:

1. Ensure that you eat nutritious diet that has less sugar, less fat and more protein

2. Moderate regular exercise. Do not strain your body

3. Strive to increase your knowledge. Learning something new keeps your mind active just as the exercise does to your body. Read a book or read it to some one.

4. As you get older try to be as independent as possible. Do things for yourself, this will increase your sense of self worth.

5. Interact with people that are positive and can bring the best in you. You also need to stay positive. Go out with friends 

6. Play games that are mind stimulating, such as chess, 30 seconds, scrabble etc. Pay attention to everything in your day, and in the evening try to recall what happened in the morning during the day and so forth.

All this can be applied to nursing an old person.

Preventive nursing goes further than only focusing on direct performance enhancement.  Among many other things, we can encourage, teach, and assist people to live in environments that promote their quality of life, whatever would be appropriate for them.

For example, elderly with diminishing performance can be encouraged and assisted to find appropriate assistance in quality assisted living environments or quality nursing facilities.

Delay the aging complications?  Does the question equate a long life without quality of life with an aging complication?  Or, does the question equate long life with poor QOL with aging free of complications?  Of course, the original poster likely meant for the question to be addressed by the first answer and PM's answer is great.  In my experience, the definition of complications changes with, among other things, one's age.  Quality of life and complications take on new meaning for old people and people in general who have lost substantial functionality or are still aware that they are losing functionality.  This answer is not meant to support euthanasia; it is meant to respond to each individual's understanding of what is a "complication" of aging.  Thanks for the question.

That is a vast question- what disease area, what specific age range? Ageing is a natural process and is inevitable, and as such preventative measures will only be effective to a point.