As with any breed, a Chihuahua's lifespan varies depending on a number of different factors (see below for a complete list). Some of these factors are controllable, whereas others are outside of owners' control. 
Owners should familiarize themselves with these factors so they give their Chihuahua the longest, fullest life possible.
There's no fool-proof method for predicting exactly how long they can live, but current data suggests that Chihuahuas live an average of 15 to 20 years when properly cared for by a loving owner.
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Small breeds like the Chihuahua and Yorkshire Terrier live longer than medium-to-large breeds like the Labrador Retriever and Mastiff. Medium-to-large breeds have an average life expectancy of 10 to 13 years.
Does this mean all Chihuahuas live between 15 and 20 years? Not necessarily, but this is a good rule of thumb to follow.
Diet – a Chihuahua's diet will directly impact their lifespan. Owners should feed their furry four-legged friends a nutritious, well-balanced premium dog food with no fillers, preservatives or artificial flavors. Avoid the temptation of feeding your Chihuahua leftover dinner scraps, as this can lead to obesity, toxicosis and other health problems.
Weight – equally as influential in a Chihuahua's lifespan is weight. Obese Chihuahuas are prone to a variety of adverse health conditions which may lower their average lifespan, including diabetes and cardiovascular disease. Underweight Chis may also experience a lower average lifespan due to malnutrition and a weakened immune system.
Healthcare – it should come as no surprise that healthcare plays a role in a Chihuahua's lifespan as well. Taking them to the veterinarian for routine checkups will promote a long, healthy life by diagnosing illnesses early, before they progress to life-threatening ailments. Most veterinarians recommend taking adult Chihuahuas between the ages of 7 and 10 for a wellness check once a year. If you have senior Chihuahua over the age of 10, however, you should take them for a wellness check twice a year.
Genetics – "good genes" can lengthen a Chihuahuas lifespan, which is why it's important to pay close attention to a pup's parents if you're looking to adopt. If the parents are healthy, energetic and happy, their pups are more likely to live a long and healthy life.
Spaying/neutering – Chihuahuas that are spayed and neutered tend to live longer than those that aren't fixed. Neutering a male Chi before the age of year will decrease their risk of developing testicular and prostate cancers, and fixing female Chis before their first heat cycle can reduce their risk of developing uterine and ovarian cancers. Of course there are other health benefits associated with spaying and neutering a Chihuahua, such as lowered aggression levels, reduced risk of running away, and fewer pups ending up in the pound. Vaccinations – Chihuahuas require vaccinations and follow-up booster shots to protect against potentially life-threatening disease like the parvovirus, Leptospirosis, Bordetella bronchiseptica (kenel cough), canine influenza, distemper and rabies.
Oral Care – like most small breeds, Chihuahuas are prone to dental problems like tooth decay, tooth toss and gum disease. Some studies suggest that as many as 9 out of 10 Chihuahuas will experience dental problems. Allowing your Chihuahua to suffer from poor oral health can reduce their lifespan. Bacteria enters the bloodstream through bleeding gums (caused by gingivitis) and goes straight to the heart and kidneys.
Gender – statistically speaking, females live longer than their male counterparts. The exact cause of this longer average lifespan is unknown, but studies suggest female Chihuahuas live 1 to 2 years longer than males.
Activity Levels – keeping your Chihuahua active is crucial to promoting a long, healthy life. Whether it's playing fetch, tug-of-war (with a toy, of course), or going for a walk around the neighbor, give your Chi plenty of daily exercise. Doing so will burn excess fat, build muscle, strengthen their immune system and ward off illness, of which impact their average lifespan.
The world's oldest Chihuahua on record (keyword being on record) is Megabyte, who passed away on January 1, 2014 at the age of 20 years and 265 days. Other than the occasional news article and blog posts, details are scare about Megabyte's life; however, we know this full-bred Chihuahua lived 20 full years before passing away on New Year's Day, 2014.
There have been several reports of Chihuahuas living to 20, 21 and even 22 years of age. These reports were never verified, and thus Megabyte retains his title as the world's oldest Chihuahua.
*this chart can be used for all dogs under 20 pounds.
