Google provides the following definition of a superfood:
"A nutrient-rich food considered to be especially beneficial for health and well-being."A fairly generic definition. Most whole foods are nutrient-rich, containing plenty of vitamins, minerals, and phytochemicals. A phytochemical is not considered an essential nutrient, but does show some promise when it comes to improving health and longevity.
Related: NAD+ Booster NMN May Fight Aging
I wanted to put together a list of foods that prevent premature aging. I hesitate to call them superfoods, but in the truest sense these edibles are just that. They will work to improve cell health, cognitive function, and fight decay at the genetic level.
But what does this even mean? Let me explain...
Obviously cell health is important. A healthy cell will live longer and help you to look, feel, and perform better. But to prevent premature aging, we also need to dig deeper. So with that said, let's talk about mitochondria.
Mitochondrial dysfunction contributes to early aging. It is also a major contributor to many popular, and extremely malicious diseases. So by strengthening cells, and the mitochondira contained therein, we are fighting off the aging process.
Foods That Help Prevent Premature Aging
#1 - Avocado
You want me to pay extra for the guac?
Avocados. We all love them. Well, except for the alien among us who believe these fruits look too much like seedling eggs. Think of the pods from Ripley's Alien series.
Avocados contain a wealth of good nutrition and healthy fats. They are very rich in potassium, vitamin E, vitamin K1, vitamin B9, vitamin C, and more. But here's the interesting part...
Avocados help protect your mitochondria from attack. Oxygen molecules found in mitochondria can be converted into free radicals. Free radicals are the enemy of good health. They will attack surrounding normal molecules, including proteins, lipids, and even DNA, and convert them into free radicals.
This destructive replication leads to disease, including hypertension and diabetes, along with premature aging.
A recent feature in the Journal of Diabetes Research explored the impact of avocado oil on mitochondria health.  One of the major characteristics of diabetes is mitochorndria dysfunction. Researchers found that avocado oil hinders impairment of mitochondrial respiration.
Yeast mitochondria are inherently resistant, but not impervious to free radical damage. This is due to a specific fat that is contained in its envelope. This same fat is found in avocado oil. Not only this, but avocado oil contains several important plant pigments that also work to inhibit oxidation, fend off free radical develop, and curb premature again and disease.
Christian Cortés-Rojo, from the Instituto de Investigaciones Químico-Biológicas, Mexico, explains:
"These results could be attributed to the fact that avocado oil caused accelerated respiration in mitochondria, which indicate that the use of nutrients for producing energy for cell functions remains effective even in cells attacked by free radicals and that mitochondria itself could produce little amounts of damaging free radicals." 
#2 - KaleBy now, you are probably all kaled out, right? Everywhere you look it's kale chips this, kale shakes that. Kale has been mentioned as a superfood so often that you probably stopped listening. Well it may be time to listen more closely to the kale hype again.
Kale is not only one of the most nutrient-dense foods on the planet, but it's also excellent in the battle against premature aging.
Kale also works to keep the MTHFR gene healthy. MTHFR is short for methylenetetrahydrofolate reductase. It is a gene as well as an extremely important enzyme, playing a critical role in the processing of amino acids.
Also, MTHFR plays an essential role in the production of folic acid. Folic acid, also known as vitamin B9, is a substance that can't be produced in the human body. Here's where it gets interesting...
Folic acid is required to process homocysteine. Homocysteine (tHcy) directly impacts cell health. As we age, homocysteine levels in our blood tend to elevate. When levels in the blood stream are higher than 8-15 ?mol/L, there is an increase in risk for certain cancers, cognitive dysfunction, blood clots, cardiovascular disease, and many other chronic diseases. 
Without a proper amount of folic acid, homocysteine is not processed properly. This leads to an increase in total homocysteine blood plasma levels.
When the MTHFR protein is function properly, folic acid is produced at an optimal level, and homocysteine levels are kept under control. This keeps cells healthy, and curtails premature aging.
The MTHFR gene can mutate, impacting folic acid production and homocysteine levels. Kale works to detoxify the MTHFR gene, helping to keep you young. This is done by optimizing methylation.
Kale contains a bounty of micronutrients required for proper methylation. Methylation is a biological process that occurs in your body billions of times per second.  Read that last sentence back slowly. Methylation works to repair DNA, and to control homocysteine.
Long story short, by eating kale you bolster the methylation process. This fights off aging by not only repairing DNA, but also by keeping the MTHFR gene healthy and without mutation, thus maintaining normal blood homocysteine levels.
Phew! that was a lot to digest. Eat your kale, kids.
#3 - Green TeaGreen tea, the ultimate Japanese elixir of health. Made from unfermented leaves, green tea contains an extremely high level of polyphenols and antioxidants. Both have the ability to battle free radicals, and the rampant damage they cause in the body.
The antioxidants that green tea contains attack free radicals. The free radical theory of aging asserts that many of the processes that contribute to aging are caused by free radical damage. These free radicals, when unchecked by antioxidants, work to deteriorate DNA and protein cross-linking. 
Polyphenols are abundant antioxidants, especially in green tea. The benefits of polyphenols are as follows:
- Work to naturally lower blood pressure
- Yield an improvement in intestinal flora, or the good bacteria that aid with digestion
- Assists with cholesterol and triglyceride regulation
- Works to reduce inflammation
Green tea contains a flavonoid called epigallocatechin-3-gallate, or EGCG. EGCG is able to bind to beta-amyloid proteins, preventing development of unwanted plaques and conditions such as Alzheimer's. 
One specific study revealed that those drinking green tea experienced a notable increase in brain activity.  Interestingly enough, the greater the amount of green tea ingested, the greater the increase in brain activity.