Immortality. Is it within our reach? Can science dramatically extend human lifespan?
One of the first barriers that must be overcome for this to happen is to surpass the infamous Hayflick Limit, which is a practical limit on the number of divisions that a cell can undergo until it becomes a weak and doddering "senior citizen", a.k.a senescent. Senescent white blood cells, for example, double one's risk of cancer according to one study.  Still another study showed that bone loss  and fertility are correlated with telomere length. Other examples abound.
The reason for the notoriety of the Hayflick limit is that most cells, as they divide, lose just a little DNA off the end of their chromosomes, which is named a telomere. Stem cells, though, overcome this shortening and, because of it, are cells that are practically immortal. How do stem cells achieve the fountain of youth? They do it, it turns out, through telomerase, an enzyme that essentially replaces the missing DNA on the end of the gene. In other words, increasing telomerase levels makes it theoretically possible that one's cells never shorten and reach "old age".
As you might expect, there has been considerable research on telomerase and the summary is this: natural increases in telomerase have shown considerable promise, whereas artificial increases in telomerase have led to increases in cancer rates. For example, one study used cancer resistant mice and then gave them extra telomerase and, indeed, the lifespan of the mice was increased.  In other words, if you can limit cancer, you can likely increase lifespan.
This could perhaps be accomplished through some of the lifestyle changes that I recommend in my Cancer Prevention link. In other words, a lifestyle that both reduces cancer risk and increases telomerase may be ideal. This is indicated in a study of very old Jews, average age 97, that showed that these centennarians had significantly longer telomeres. 
NEWS FLASH: A groundbreaking 2010 study in the journal Nature reported some potentially incredible telomerase-related news. Scientists created mice that aged very quickly by developing a line with low levels of telomerase. What was exciting was that when these mice had their telomerase reactivated through a clever chemical process, the mice regained their fertility and brain function and all without cancer.  Of course, this is just an initial animal study, but it is a very promising first step.
So how can we increase telomerase in a (very likely) safe way in order to keep our telomeres nice and long, the way nature intended them? Here are some of the key strategies from the latest research, which, you may notice, match many of the healthy strategies for your heart, brain and skin:
1) Fish Oil. Fish oil reduces oxidation of certain key metabolic processes and this is likely the reason that those with the highest levels of omega-3's have the longest telomeres. 
2) Ornish Diet. As we have mentioned elsewhere, the Low Fat (Ornish) Diet slows down telomere shortening. And, of course, a Low Fat (Ornish) Diet will actually clear the plaque right out of your veins, decrease erection-killing blood pressure as well and increase your telomerase activity. 
3) Sitting. The difference in telomere lengths between the most active and least was equivalent to about ten years of aging according to one study.  Sitting isn't just hard on your bod - it's hard on your chromosomes.
4) Exercise. Exercise may be king when it comes to slowing down telomere aging. Older runners have telomeres 75% less aged than their sedentary counterparts.  Research has shown that exercise activates telomerase.
5) Vitamin D. Lower vitamin D levels are associated with shorter telomeres, at least in women. 
6) Stress and Cortisol. A number of studies have shown that stress shortens telomeres. For example, one small study showed that women sexually abused as children had shortened telomeres, i.e. not only were they scarred emotionally but on the cellular level as well.  The reason appears to be tied to the stress hormone cortisol, which has been shown to reduce telomerase activity.  A further verification of this was recently found in a study of women with chronic anxiety, who were found to have shorter telomere lengths. 
7) CAUTION: Green Tea and Curry/Turmeric/Cursumin. Green Tea is a little scary. The key ingredient in green tea, EGCG, has been found to be a powerful telomerase inhibitor. And, in some cases, this likely gives green tea its anti-cancer properties. However, do you want cancer protection simply because your chromosomes have been shortened to senescence?  Turmeric has the same property and therefore I am cautious about using this as well, even though I love curry! 
8) Smoking. Smoking attacks just about every system in your body so why not your chromosomes as well, eh?  This was verified in a study that showed that telomere length was actually affected in a dose-dependent manner, i.e. the more pack-years the person had smoked, the shorter their telomeres. 
9) Overweight. That spare tire around your middle likely accelerates aging according to one study (of women), which found that the more the weight and the higher the BMI, the shorter the telomeres. 
1) Cell, Nov 2008, 14;135(4):609-22, "Telomerase reverse transcriptase delays aging in cancer-resistant mice"
3) JAMA, Jul 7 2010, 304:69 - 75, "Telomere Length and Risk of Incident Cancer and Cancer Mortality
4) Journal of the American4) Journal of the American Medical Association, 2010, 303(3):250-257, Association of Marine Omega-3 Fatty Acid Levels With Telomeric Aging in Patients With Coronary Heart Disease
5) Mech Ageing Dev, 2005 Oct, 126(10):1115-22, "Telomere length versus hormonal and bone mineral status in6) Biochem Biophys Res Commun, 1998 Aug 19, 249(2):391-6, "Telomerase inhibition, telomere shortening, and senescence of cancer cells by tea catechin"
7) Arch Intern Med, 2008, 168:154-158, http://pubs.ama-assn.org/media/2008a/0128.dtl
9) The American Journal of Clinical Nutrition 86 (5):1420 5, "Higher serum vitamin D concentrations are associated with longer leukocyte telomere length in women".
10) Biol Psychiatry, 2010 Mar 15, 67(6):531-4, Epub 2009 Oct 14, "Childhood maltreatment and telomere shortening: preliminary support for an effect of early stress on cellular aging"
11) Lancet, 2005, 366:662 664, "Obesity, cigarette smoking, and telomere length in women"
12) European Respiratory Journal, Mar 1 2006, 27(3):525-528, "Telomere shortening in smokers with and without COPD"
13) Brain, Behavior, and Immunity, May 2008, 22(4):600-605, "Reduced telomerase activity in human T lymphocytes exposed to cortisol"
14) Cancer Epidemiology, Biomarkers & Prevention, Mar 2009, 18:816, "Obesity and Weight Gain in Adulthood and Telomere Length"
15) Nature, 2010, Received 08 May 2010, Accepted 26 October 2010, Published online 28 November 2010, "Telomerase reactivation reverses tissue degeneration in aged telomerase-deficient mice"
16) The Lancet Oncology, 9(11):1048-1057, "Increased telomerase activity and comprehensive lifestyle changes: a pilot study"
17) PLoS ONE, 2012, 7(7):e40516. "High Phobic Anxiety Is Related to Lower Leukocyte Telomere Length in Women"
18) Cancer Letters, 8 Oct 2002, 184(1):1-6, "Curcumin inhibits telomerase activity through human telomerase reverse transcritpase in MCF-7 breast cancer cell line"